The tarp that i carry now is the Dana Designs Hat Tarp.
http://tinyurl.com/sbouj
a tarp that has a zip running around the perimeter...string it up like a tarp when needed off of a tree and a couple of paddles...
OR zip it up-it folds in half like a taco.....1 side is folded material...two sides are zip...third side is open....climb inside...space for two people....waterproof fabric keeps you warm and dry....
this might be helpful too: http://www.hilleberg.com/Catalog/bivanorak_4846109.htm