I saw these cosmetic pouches with waterproof lining at TGR/Tiger today. They cost £3/€4 and would fit a personal Filofax or smaller. My enormous stuffed VDS with 30mm rings is super fat and it did not quite fit in but anything of a reasonable size should slide in perfectly.
I have photographed the pouch on top of my VDS personal sized binder so you can get an idea of size. They were really nice! I am going to get one tomorrow.