Head back to this thread and look at the link for thermal wrap. It is a TyVek product that is also reflective. It is for use as a radiant barrier as well as a vapor permeable air barrier for construction. Turn the shinny side in, and you have a tarp/tent/blanket that is water "proof" and reflects heat back.

Contact an insulation contractor to see if you can get a remnant roll. I think the rolls come as long a 12 feet.