If I were you, I would invest in an Outdoor Research Advanced Hydroseal Stuff sack. They're not that expensive ($30 or so bucks), you can put other things in it (sleeping bags, etc.) its made out of coated nylon (so you can compress it down easily), and they come in several sizes. I have put all kinds of things in there and while I might not be able to drop it in a lake and have it dry, it will definetly take on a rainstorm without a problem. I've never had a wet sleeping bag with them before.