Mayan, might I make some suggestions? Sunforger or boat/marine types of canvas. They are a bit stiff, quite waterproof, and can easily be decorated.
TYVEK is a house-wrapping useful for a lot of waterproofing needs, you might use a pretty fabric, and line with TYVEK? I just ask construction crews for a few scraps, they can't use anything much smaller than a 10 foot length.
For a stiffener, styrene from a hobby shop is a thin, light plastic which takes glues well and is thin enough to be sewn with a canvas needle on a heavier duty machine. It could also be slipped into the space between the liner and the outer shell, once an item was half-constructed.
If you choose to make snack bags, I'd be thrilled!

