Did you ever look into the feed sack material?
It's lightweight, waterproof, durable, and you can sew it.
Hmmm. I wonder just how well it can be sewn with a normal machine? I know it comes sewn on the top but that's very heavy "thread" and only 3-4 stitches per inch. May have to try it someday since I've saved a few dog chow bags.