If you don't want to spend a bit on a pack ( the Bison gear Lost River has a snap out liner that is supposed to be waterproof), go to a outdoors store and buy a smaller waterproof canoeing gear bag. Stuff it into the pack you already have.
I have extended my pack capacity by putting a good quality game bag inside, filling it with meat right thru the top of the pack and tying it off to the straps. This is what the Lost River does, using a more water proof bag, and what you can do using the canoe bag.
Chuckc