Get yourself a surplus poncho and liner and strap them on the top or bottom of the bag, the combination is much more versatile than commercial ponchos and "survival blankets", and a lot tougher too. Poncho and liner can be found at most any surplus store, and if you don't have one of those near by, try Cheaper than dirt or Brigade quartermaster. Once you spend the 30 or 40 bucks and have them in hand, I'm sure you'll agree it's money well spent.

Troy