Start by measuring all of the dimensions, taking care to consider all of the following:

How will the shroud attach to the radiator?
Are there any significant obstructions?
What's the distance between the fan and the radiator?
Are there any noteworthy angles or off-center components?
How much clearance is needed aroung the fan to allow for engine or body movement?
For example, this fan is off-center towards the passenger side by about 1/2", sets back about 1 3/8" ,and it swings about 1" above the top of the radiator. It is parallel with the radiator, and there are no hoses or other obstructions to deal with on this particular project


Constructing the foam buck
Using a piece of thin plywood is a good way to start on any part that has a flat mounting surface. The plywood provides a sturdy base upon which to lay out and construct the foam buck. It will also serve as the form for the finished fan shroud's mounting flanges.

I had a bunch of 2 lb polyurethane foam left over from other jobs, so I thought this project would be a good place to use some up. It also can illustrate how easy it is to use this foam -- it cuts so easily and shapes so well that piecing it up doesn't add all that much time to the project. This foam is available in thicknesses from 1/2" up to 6" thick at any fiberglass supply house. You can also use the polyurethane insulating foam that is available from building supply stores. Just peel the foil layer off. (Keep in mind that styrofoam won't work under any resin with styrene in it, unless you can bar the resin from it.)


Shaping and gluing the foam
Cut foam to shape with a handsaw, putty knife, or bandsaw. Then, fit and glue pieces of foam larger than your layout on the board. A hot melt glue gun makes gluing this foam fast and easy. You can also glue the foam down with an expanding urethane glue, such as Gorilla glue, or some of the expanding foam that comes in an aerosol can. The different glues can make sanding over seam lines difficult, as they are usually a different density and hardness than the surrounding foam.

Shaping this foam also can be done with sandpaper, sureform files, knives, and picks. Even your fingers can shape this foam quite easily and rapidly.

Keep the hot melt glue away from where you will be sanding. It is gummy and will not sand properly, making your buck a lot harder to shape than it needs to be.

 
The initial pieces of foam are cut and glued down to the plywood base. 


For the round part of the shroud, a circle is scribed in the foam.


More pieces of foam are cut and glued along the scribed circle

The glue won't sand properly, so keep it off the areas you will be sanding.
Forming the circular part of the shroud
It's often the case that part of the shroud will need to be perfectly round. If so, set up the buck with a bolt or nail in the center, and fix it to your drill or mill table so that it can be spun by hand against an end mill. (A drill bit or drum sander works well also.) This will make it easy to shape the circle, as well as cut the box section down to its proper thickness. There are many other ways to shape your buck. You are only limited by your imagination here.

You will notice a small ledge left at the intersections of the vertical and horizontal surfaces. This will be sanded into a radius by hand during the final sanding.

The plywood that is still sticking out from under the foam will be used to form the flat mounting flanges.

Use a sanding block with 40 grit sandpaper to taper the edges of the plenum box down.

A bolt centered in the buck allows it to be spun against an end mill to shape the circle.

The buck is spun by hand to form the circular portion of the shroud shape.

I made a small miscalculation in the fan tube size and had to add some foam around the edge. The final step in shaping is to radius the square outside corners with sandpaper


One step that I do on these kinds of parts is to paint the buck with resin after shaping it. It makes it a little more dimensionally stable, and toughens up the surface for the next steps.

Once I have painted the buck with resin I can clay up any low spots with modeling clay, and then wax the resin. I then spray a coating of polyvinyl alchohol on the waxed surface. Polyvinyl alcohol, or "PVA", is a water-soluble plastic commonly used as a release and sealing agent for molds. Using PVA will make removing the foam much easier after the part is laid up.

When shaping is complete, the buck is painted with resin

Laminating the part
To laminate the part, you will need the following:

china bristle brushes
acetone
cans for mixing resin and holding acetone for washing your hands and tools
scissors
resin
catalyst
fiberglass mat
cardboard
latex gloves
This article assumes that you have a basic knowledge of laminating with fiberglass. Follow all instructions provided with your materials in regard to proper catalyzation of the resin and storage and handling of your tools and materials.

Cut strips of 1 1/2 oz fiberglass mat on a peice of cardboard, ready to be wetted out. 3 layers of 1 1/2 oz mat will make a part that is about 0.120" thick, as each layer contributes about .040" to the laminate. There will be an extra layer laminated around all of the edges to make the final part about 0.160" thick.

When wetting out mat with resin, many people work the resin into the mat while it is on the wet out board. This is a mistake and will only make a resin rich, inconsistent part. You only brush enough resin on to cover the mat with resin. Then, give it a little bit of time, and it will soak in. Notice in the picture how there are some areas of mat that are darker, and have become transparent. There is no more resin on those strips than is on the others, they were wetted out earlier and have soaked up the resin.

You do need to be somewhat swift with this part of the laminating process. As time goes by the resin will dissolve the binder that holds the mat together and make picking it up to apply it to the part a very frustrating activity. If the binder has dissolved, the mat will just fall apart when you try to pick it up.

Cut strips of 1 1/2 oz fiberglass mat, and lay them out on a piece of cardboard, ready to be wetted out

Work quickly but carefully, as the resin will gradually dissolve the binder.
Brush some resin on the surface of your buck. Brush a bit more resin in the inside and outside corners, as these areas tend to be a little dry after stretching the mat over them

Brush the resin onto the surface of the buck. Pay special attention to the corners

Pick up the material from the wet out board and apply it to the part. Don't try to push it into any inside corners or stretch it over any outside corners just yet. Just get the material stuck to the major flat areas.

Stretch the material over any outside corners, but don't try to fold it as it will only spring back. Push the material into inside corners, stretching it as you go. If it gets thin, more material can be added. I usually stretch the material, and wind up with a section of mat that I can reapply to the stretched, thinned out areas.

As shown here, hold the material down with one hand while you stretch it over an outside corner with the other. If you pull some material off the edge, you can put it back over the outside of the corner, as it has probably gotten a bit thin there anyway.

Apply the wet out fiberglass board to the part.

 made a roller out of some threaded rod, a file handle and some 1/4" rod. It works extremely well for rolling out air bubbles in fiberglass. Move slowly, applying enough pressure to roll the air out, but not so much pressure that you move the mat around or push all of the resin out. Air bubbles look like clear voids, they are easily spotted when you put pressure on them as you can move them around."Dry" spots will show up as white strands of fiberglass that spring up out of the resin. Use consistent, firm pressure, moving steadily. It works better to pull the roller than to push it.

You can also use a brush, foam roller or your fingers to manipulate the air out. Squeegees don't work well on fiberglass mat. Commercially made rollers of many sizes and shapes are available from most fiberglass supply houses.

A roller can be made from threaded rod, a file handle, and 1/4" rod. 
You can see the difference between a rolled-out area with no air, and an area that still has air in it. 
When you roll into a corner, dwell there with the roller for a second, to allow the air to come out of the corner
Roll over a corner, not parallel with it. 
Now all the air is rolled out.

Cut your material so that you have at least 1" of overlap onto the previously laid up section. Stagger the layers so you don't create a big lump. A trick that professional glass guys use is to tear the edges of the mat. This tapers the thickness down at the edge, making a smoother overlap. Done correctly and applied while the previous lay up is still workable, you won't be able to tell that the part was made from separate pieces of mat, instead of one continous piece.

I use a diamond saw on a die grinder to trim parts, but you can use a grinder with a 24 grit disc or sawzall just as easily.

Fiberglass is very messy, and the dust is quite a nuisance, as shown by the dust that has accumulated on our test dummy. Though fiberglass dust and fiberglass strands are generally not considered to be carcinogenic (except in cases of uncommonly high exposure), they are widely recognized as skin, eye, and respiratory irritants. Use of a minimum N95 dust mask, and proper eye and body protection is a must. Don't do this in your house, or anywhere without adequate ventilation. The dust will migrate everywhere if you don't exhaust it, or collect it. For more information on the health factors associated with fiberglass,


Trimming off the excess with a diamond saw on a die grinder. 
All trimmed, ground, and ready for some body filler. 
Use a dust mask, and proper eye and body protection when working with fiberglass dust

Finishing the part
After the part has been trimmed and the surface has been lightly ground, with any high spots ground down, and any air bubbles opened up, a skim coat of body filler can be applied. Any good body filler that will stick to fiberglass will work. Scrub a light coat into the surface in 3 or 4 directions before applying the fill coat, this will get the filler into the pores better. The part is still left on the foam and plywood buck because it is easier to handle during the bodywork phase.

As this is a one-off fan shroud, special fairing techniques are not employed in its execution. A DA sander with 80 grit is used in all accessable areas, and hand sanding in the corners is done with 80 grit as well

Leave the part on the buck for ease of handling. 
Apply a skim coat of bondo. 
Some areas need another coat to level the surface.

After sanding with 80 grit, a final sanding can be done with 120

Now the foam buck can be chiseled out. It is very soft and offers little resistance to removal. The wax and PVA that were applied to the buck are making it easy to separate the foam from the part. Most of it just pops right off, but a blow gun can be used to remove the smaller flakes.

Chiseling out the larger pieces of foam. 

Clean inside and out

As mentioned before, this shroud is going to need to enclose the radiator where the fan swings 1" above the top. This will require a secondary lamination to form a return. As the return can be flat, and on the same plane as the mounting surface, a table top will be used for the mold. I simply tape the top off with some masking tape, then wax the tape with some mold release wax. I like Partall #2, AKA "green wax".

Just clamp the shroud to the table, and lay up 3 layers of fiberglass mat.

use polyester surfacing primer on all of my plugs and one-offs. Evercoat-Fiberglass Co. makes a material called Featherfill, Poly-Lux, Inc. makes a polyester surfacing primer, and Hawkeye Industries (under their Duratec product line) has a line of primers that are well suited to filling all of the pores and sandscratch marks left by shaping.

I like the Duratec surfacing primer the best, because you can build up to .040" in a single application

Nothing fancy in the way of spray guns here. The primer was sprayed with a very old Devilbiss

Now that this part is primed with the polyester coating, it can be sanded and top coated with whatever paint system you like.

Also, if I was going to make several of these parts, I could sand and polish the surface, wax it, and use it to create a mold with which I could duplicate it many times over...