When the plastic begins to change shape and to lower, press it under the shape. Okay, this is how it looks like, notice the stretched holes, that's actually an accident, because the plastic was too close to the light, but notice how this could be used quite efficiently to create a really rusty air intake!
I tried different ways of heating the plastic -under the lit gas oven I only managed to burn myself while holding the thick (1,5mm ) plastic sheet, under one single flame, icouldn't manage to get enough plastic melting! So i finally got round this stupid problem by holding the sheet of plastic under *4* different candles!
back again for good. Well, 6 pictures for such a small work might appear a bit stupid, but i have them so... those intakes where done from surprisingly thin metal, and their peculiar shape led me to no other choice than to vacu-form the shape in 2 parts. The first part was to build a master. I then cut 2 plastic rounds to the right dimensions of the upper and lower parts and used some Magic Sculp to sculpt the shape, I then sanded the final master.