Since I'm doing a custom build, I decided I wanted a hood scoop on the spare hood that I had. With detailed instructions from Len Stuart of Autobodystore.com, I settled on a Shelby OT scoop.
The rest of this section is Len's instruction (with my comments).
Take the scoop and place it in the desired position and draw around its outline on the hood.
Cut the metal for a functional scoop so that you have a couple of inches under the edge of the scoop.
Drill some 1/8" holes in the scoop through to the hood about every inch and insert a screw to pull the scoop down. Doing this type of mounting on other areas of the vehicle you may be able to clamp the panels together.
(Earl: I didn't drill every inch but your needs may vary.)
Grind the metal clean near the point of contact as well as the hood scoop using a 16 or 24 grit disk.
Apply your bonding material - I've been using Dyna-Weld - to both surfaces pushing the material into the scratches then build up a little on the surface and put the two pieces together and screw it down. DON'T screw it down tight because the Dyna-Weld is your glue and if you squeeze it all out your scoop will pop right off. Just pull the scoop down enough so that the paste starts to push out of the crack a little.
(Earl: Since the Dyna-Weld was a one-to-one mix ratio product, I used 2 matching caulking guns to dispense. A number strip comes iwth the Dyna-Weld kit to apply to the plunger handle of the caulking gun for more accurate dispensing.)
After the material gets hard remove the screws and grind off the excess bonding amterial and do some initial shaping with the grinder.
Countersink the screw holes and cut a strip of fiberglass about and inch wide to cover the seam all the way around the oustide of the scoop.
Using more Dyna-Weld (DW) apply it to the edge filling the screw holes and giving you some buildup over the seam then push the fiberglass cloth into the soft paste so that it covers the seam and apply more DW on top while everything is still soft.You may need to do about a foot at a time because this could take long enough for the DW to start to harden. So cut the cloth strip into 1 foot lengths and do one at a time until you're all the way around and the cloth is buried in the DW and covering the seam.
Once the DW is hardended over the cloth use the appropriate sanding block to shapre the filler. Apply more of the filler (you can now use flexible polyester putty) and continue the shaping but try not to sand so much off that you sand through the cloth reinforcing strip.
(Earl: This picture shows some of the tools I used for the shaping and sanding of the material to create a smooth transition between fiberglass and metal.)
The idea here is to use a flexible material (Dyna-Weld and flexible putty like Poly-Flex) so that the different expanding/contracting characteristics of the materials won't cause cracking.
When you have your shape sanded to where you're happy wtih 80 grit paper then go over it with some 180 and then spray it with some 2K primer using a flex additive so that the primer is also flexible. After guide coating and sanding the primer wtih some 400/600 grit sandpaper you can apply paint but be sure that the paint ALSO has flex additive added so that it won't crack.
(Earl: Picture here is the finished hood covered with Kirker Euro epoxy primer. I really like this primer and it is easy to sand when necessary.)