I agree with JimmyD - buy a roll of silverized mylar - I got a narrow (54") one quite cheaply from Porters some years ago, and I know you can find it 10' wide on the web.
You can try the "hang weight and apply heat" - but I don't think it'll work well on Mylar. That plastic is highly cross-linked and pretty resistant to heat, so it shouldn't "flow" easily. I just did a Scirus search for "Mylar temperature limits" and found a report at the Oak Ridge National Lab that describes heating mylar film at 900 C to seal it into an alumina crucible. 900 C is about 1600 F, not so hot as the hottest part of your gas stove's flame, but pretty darned warm, indeed. OTOH, it's a cheap test.
You can probably find other metallized films nowadays, as well. I've seen a partially silvered holographic film - irradescent, like flashes of rainbows - somewhere.