Came across this on one of my forums, and thought I'd share. Basically you fuse plastic bags together with an iron to create waterproof material you can sew together:
Once I get done moving, I think I'll be building a cheap new waterproof pannier to test the usefulness. This might also make cheap waterproof clothing for when you can't find quite the right size or shape to fit something. Seems like this has a lot of uses for the more crafty.