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.
I just bought cat litter for my babies, I'll have to look into it.
5x7 Eastman-Kodak kit / B&L 135mm Zeiss Tessar + Compur Deckel
4x5 Graphic View, made in USA! New project
RB67 Pro S /50 4.5 / 90 3.8 / 180 4.5 / WLF / prism finder / polaback
Random 35mm stuff