I found a site that sells a "survival sleeping bag" made out of the mylar foil type material that the survival blankets are made from. It looks like a big step up from the survival blanket in that you don't have to try to hold it closed around you. You can poke a hole in the closed end if you need more ventilation.

Here is a site that sells them for $20 for ten bags.
http://www.mcrmedical.com/category/space-blankets.html?gclid=CJ3ko-KikZsCFQq3sgodCR7koA

David Enoch