I have a home remedy type of recipe for waterproofing canvas. I have never tried it so I can' tpersonally vouch for it's effectiveness. It's oil based so it may mar your colors and leave an unpleasent scent or feel to the bag.

3c. soybean oil
1 ½c. turpentine

Mix and paint or spray on canvas
Repeat as necessary if exposed long periods of time

dust with talc or powdered chalk