Are you making a lens or a bucket? Abs without some sort of flame ratings is very toxic when it burns.
If you are making a lens...
There are a few different grades of lexan, some with heat resistant coatings on them. You can get glass filled polycarbonate up to 300degrees fairly cheap and it comes mirrored, or tinted or clear, etc. What kind of bulbs are you running? You definately want a material that is flame retardant thought. Also look into Acrylics. Both can be prouduced translucent, also with tints. Both can have a flame retardent rating. Just figure out what temp your bulbs are going to burn at.
you may want to check if the material can be thermal set. I know from experience that you can heat lexan and bend it to a certain point(not far) and it starts to crack interstially.
You may want to check a company called smooth on(
www.smoothon.com). They make 2 part plastics to cast your own lenses. You would just need to make a mold. They have starter kits for this i believe for $20-$30. Would be enough for one. These kits are meant for tail lights but should work okay i think.
If your trying to make a bucket i would just fiberglass it. but smooth on should also have what you need for that as well. I would stick to polycarbonates if you are trying to thermal form something.