Molding grill blocks
 

I would like to improve on the coroplast grill blocks (valence, foglight blocks) that I have on my car. The coroplast is pretty good, but there are gaps around it and it is not flush with the rest of the surface of the car.

I would like to build a mold and cast fiberglass parts that will fit in the openings and look like they are stock.

My first idea for making a mold is to drape a thin plastic (saran wrap) over the area to be cast (for example the foglight openings) and lay fiberglass soaked in epoxy on the opening. Hopefully it will stick and not fall off, since it is vertical.

Are there any other materials which are easy to use to cast a mold for an opening like a valence or foglight opening?

I was thinking plaster or paper mache might be other materials.

You could use spray foam to fill in, then sand, putty, and paint. I'm (still) working on my foam upper grill block, which I'll cover with fiberglass before painting so it'll be more rock and debris resistant.