With the intention of teaching myself to work with fiberglass, I upgraded my coroplast radiator block to a fiberglass grille block this evening. I'm learning as I go, but feel free to speed up that process with your criticism of my technique.
I decided to permanently block the entire upper grille. First, I screwed a block of styrofoam in place, driving two drywall screws into the plastic bumper cover. There may have been a way to do this without putting holes in the bumper, but I don't plan to un-block the upper grille, and screw holes are pretty easy to patch anyway.
I shaped the foam with a serrated steak knife (couldn't find a hacksaw) and a rasp. The rasp actually removes material very quickly, and the whole process doesn't have to take long. I get the feeling this is the stage where it pays to spend enough time to get exactly the right shape.
I made a small cutout so you can reach the hood latch. You can also see that I should have started with a larger block of foam.
I ordered one yard each of twill weave and regular weave fiberglass cloth. This is the twill weave, which is more fragile, but also more flexible. Regular weave probably would have worked better here.
I thought I'd experiment with a quick and dirty construction technique on the lower panel. What happens when you hang fiberglass cloth over a bumper opening or fog light well and epoxy it in place? Will it be smooth, neat, and strong, and is the epoxy enough to bind fiberglass to a bumper cover? If not, my panel is held on by three screws, and I can add more as needed.
My license plate is an aerodynamic device. It's currently in the winter position, so 1/6th of the lower grille is open. When it gets hot out, I'll move it to the passenger side, over the lower grille block, thus leaving half of the lower grille open.