Quote Originally Posted by 36 sedan View Post
My tin skills are poor, this has compound curves. I'm afraid I've wasted more on tin already than I could have made it from fiberglas.

Do I need to make a reverse mold out of plaster first, or can I lay over the plastic lid?

Are there other materials besides fiberglas I should be looking into to make this from?
If you've got a good fit with the plastic lid, and if you can accept that the fiberglass piece is going to be slightly bigger since it's laid on top of the fitting part, then simply wax the plastic with a good coat of wax and lay the glass mat on top. Let it cook off for an hour or so and it will pop off easily and you can then sand it smooth, hit it with a coat of filler and it's done.