Cool looking robot dude! Is it from a kit or totally homebrew?
Anyway, I'm no expert on vacuum forming, but a friend of mine made some Remote control car bodies using a home made vacuum former. He basically had a shop vac hooked up to a wooden box with a bunch of holes in the top like an air hockey table. Then he would put a sheet of plastic on a wood "frame" and put that in the oven for a few minutes. Then, he would put his positive mold on the box, press the melty plastic over the object, turn on the shop vac, and in like 2 seconds he would have a perfect formed mold.
If you just make a model out of artists clay/plaster, then let it dry, that should work perfect. Just make sure that it is very smooth. I would use clear plastic, PET or better yet lexan, and then just mask off the windshield and paint the inside of the body like an RC car. That way, even if you scratch the outside of the body, the paint job is protected.
If you have access to a CNC, just make a positive out of hardwood, mdf, or even signmaking foam. Anything that will be reasonably smooth.
The only other thing to figure out is how to mount the body, rather than letting it flail about. again, look at some pics of rc cars. You can get those style of mounts for less than $5 on ebay and adapt them to fit your bot.
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