Free: Contests & Raffles.
I am having trouble finding the ones I think would work online but I am sure you could find one in a boating store. They have really cheap disposable ones that are very small and light weight and could be easily added into a backpack. If it can work on the open ocean I think it could work in the back country. You just would need to really make sure the bag was sealed properly. It would really help if I could find a link to the specific device I have in mind. I'll keep looking.
Get rid of GoreTex.