I disagree with the box tape/styrofoam releasing quality. I've found that it leaves the fiberglass krinkly and misshapen that you could bondo out but not sand out.
I've made a couple speaker boxes and a cafe seat. I've also "helped" with a hull and bumper repair.
I'm no genius at it, but if you cover the frame with painters tape, followed by aluminum tape (about 6 dollars) then put carnauba wax on it it releases very easily.
I'm actually looking to start making molds for side covers and seat with the help of my friend that makes masks and props for movies, soon.