Okay, for something a little more practical, try one of these:

http://www.land-shark.com/

http://www.adventuremedicalkits.com/products/KIT_e_ess_onebivvy.htm

http://www.rei.com/online/store/ProductD...t_cat=undefined

I believe these are superior for what we are looking for. They do take up quite a bit more space than the single sheet mylar bags or a large plastic garbage bag, but then again, they are much more sturdy and more suited to the intended purpose.

They are like the HD space blanket turned into a bag. Since both models are reinforced laminated bags, you should be able to use them quite a number of times if you are careful.

One time my brother and I made a nice fort in a perk hole out in the forest behind our house. We snatched a bunch of dad's visqueen and carefully spread it over the trench hole and sealed it off except for one end to climb into. Once we were both inside the hole we pinned the flap shut with a couple big sticks. All was well, for a little while, as we were out of the rain and reasonably comfortable. After a couple hours, it started getting a little tough to breath right, and we opened the end of the plastic and stuck our heads out and realized we weren't getting much O2 inside that plastic.

Be careful about snuggling down into a plastic bag and sealing it up nice and tight to get the heat up. It may turn into a body bag.