Tidy Cat Crystals in a perforated 35mm film container are a good solution. Drill a series of 1/8" holes in the sides, slip in a liner made from coffee filter material and fill it with Tidy Cat Crystals. The crystals are pure silica gel. It will hold a lot of moisture and won't fill your cases with particles. I have no idea what other brands of cat litter may be the same, but it's worth looking in to.