Nice to see the stuff coming 'round *gg*
The thing is - the coke bottle acts like a heat shrink. You won't be able to make any vents or other small concave details with this method.
Polycarbonate sheet will just soften when exposed to heat, then you can vacuform it almost any shape you like. But it will also pick up any surface defects of your mould. The bottle material just tightens around it. For the piranha I could use a simple balsa wood mould without any filling or hardening the surface..easiest way, but limited in detail..
