[LUTNIT]

I really don't recommend lexan, it doesn't thermoform worth crap. I've done thermo forming with strip heaters, ovens, and real vacuum forming machines at school and polycarbonate (lexan) wont even bend when heated. Eventually it gets hot enough and bubbles start to form in it and it still doesn't bend too well but at that point its turning into an almost liquid. Apparently there is a special way to heat it to form it but conventional methods don't work. You could try Nylon, acrylic (plexiglass) or any of the forms of styrene (ABS is one of em and does thermoform) as they do become pliable when heated but polycarbonate goes from its rigid form to a really nasty liquid form without that nice intermediate state that allows vacuum forming to take place. Since your coating it with fiber glass you could use a softer and MUCH easier vacuum formed plastic like polystyrene (plasticard to the warhammer community) as the base since the fiberglass will add the strength.

Also plaster isn't a very good vacuum forming mold material. The air doesn't pass through it so the vacuum can't suck through it so it won't take the forum of the mold very well. I don't know how this will effect it since you'll be going slowly but I figured I'd throw it in there. If you run into problems with the mold you may have to drill some small (1/16") holes in the deep parts where air gets trapped to allow the vacuum to suck through. Because plaster is so brittle this might not be possible.