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%20Bivvyi 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!