There was a similar discussion here:

http://www.canoetripping.net/forums/forum/gear/tents-tarps-and-hammocks/64398-%E2%80%8Btripping-truck-tarp-solution

Granted, you may not have the canoes for the structure, but the reflective tarp might be worth considering. I don't know what material it is made of, but it does have a silvered side.