This is it!
Shop Pactiv 1/4-in x 50-ft x 4-ft Extruded Polystyrene Insulated Sheet at Lowes.com
I do not know when an ecomodder would want two hundred square feet of foam board, I just thought that something that thin would be great for small projects, especially if you need to bend it. A quarter-inch before fiberglass would still be too thick for my fog lights, but that could work for someone. It is cheap enough that I would use it freely.
For the record, someone made a boat tail with one-inch foil-covered foam board and he did a beautiful job bending the foam.
Honestly, coroplast might be too thick to cover my fog lights. You certainly need to use it so that the corrugations are perpendicular to the bend.
I bought a garage sale sign. I looked at signs just made from a single sheet of plastic, but I thought that paint (or Plasti-Dip) would just flake off.
Would Plasti-Dip waterproof poster board?
Well, I ended up trying something else, and learning a great deal. I will start a thread once I have gotten anywhere.