i'd suggest a heavy-duty mylar bivi bag, like the one Izzy mentioned, from Adventure Medical (retail $34)

http://www.adventuremedicalkits.com/prod...Thermal%20Bivvy

i carry one like it called a Land/Shark http://www.landshark-online.com/. i always take it along when in the woods.

and like Izzy said, maybe even better is the one they have coming out early next year for $50 (retail) that has a gortex-like fabric cover:

http://www.adventuremedicalkits.com/prod...January%202012!