I buy my kitty litter in the large, plastic jugs. When empty I cut out the large sides and use them, sometimes doubling them. I also make a "sleeve" inside, so the plastic can be removed when washed. I've also used Very heavy cardboard, usuually 2 layers, one with lines of cardboard running east and west and the other running north and south. Again, making a sleeve so it can be removed for laundering.