I shoot 4x5. I use Zip-lock bags. I put individual film holders in the larger "sandwich" size and about 5 bagged holders in a 1 gallon freezer size. I carry the bagged slides in an insulated, zippered fabric "lunch bag" .

Keeps dust out, waterproof and I can read the film designation. The plastic does not seem to develop a static charge and the lunch bag provides some protection from extreme, or not so, temperatures.