What type of plastic should I use for heat forming??

I need help selecting a type of heat-forming plastic. Can any plastic experts offer advice?

I'm nearing the end of the design phase of a project, and I need some help selecting the right material with which to make it. I'll essentially be building a solid, 3-dimensional form by setting a plastic sheet over the form and hitting the plastic with a heat gun, and then trimming.

Are there any plastics that would "droop" onto the form when heated, but stiffen up again properly when they re-cooled?  A pliable, but still fairly rigid plastic that resembles the stuff those cheap black plastic oil receivers are made out of would be ideal. 

If this type of plastic exists, where would I get it? Also, what thickness would be best? I would like it to be as thin as possible, but still function as described above. I have looked into vacuum forming, and while AMAZING, it is too expensive/difficult for my purposes. 

Any advice??

You can get these big blister pack sheets and make a suitable frame and mould for them.
Basically the sheet is secured into the frame with some screws, heated in the oven and when reaching the soft temp placed over the mold.
A vacuum sucks out all the air and forms the sheet perfectly onto the mold.
Works the other way around too if you want to use pressure to work the plastic against the inside of a mold.
If you have any bif factories around that also pack in blister packs you can try asking for one or two sheets to try as the usual suppliers only sell in big volumes but not single sheets.
Some hobby and craft shops (maybe even homedepot) sell thin plastic sheets as a splash proofing, the very thin ones could be substituted for your job too.
Biggest problem will be the heating and forming as it is never a good idea to work on bigger pieces with a heat gun.
You want the entire sheet to be soft and form it while hot, so fast and effictive, anything else will only result in a warped surface.

Any thermoplastic would work.

Acrylic "drape moulds" quite well. ABS is another possibility, or polystyrene.

Draping is easy to do in an oven