Who mentioned plexi?Everfalling said:i dont know for sure, but i'd think that the styrofoam would end up melting before you're even able to bend the plexi.
To the OP, yes, doing it over styrofoam should work. Just carve the styro to the shape you want, and put it over the top of it. For a nice finish on I would suggest a thin layer of bondo, smoothed out, and coated with release agent and then gelcoat. Apply your fiberglass and it should pull straight off when cured. Of course, to get the nice finish on the side you see you need to make a negative relief of the cover. If you're really cunning, you can use a white gelcoat and save yourself the effort of painting (assuming your mold is good enough).