The plug was a 2 inch thick sheet of insulation foam board that I carved out and used bondo over that till I got the shape I wanted. Primered it then used a molding wax then mold release and made my mold. The plug was pretty much toast after that. No shrink wrap it's not carbon fiber.
or something that I wanted to show the fiberglass cloth and it's pattern. Took many hours and money to get to be a spoiler. My dad use to make fiberglass dune buggy's when I was a kid so learned a bit from him but really I am far from being an expert. Just enjoy building things.