An aloksak bag or even a simple ziploc would work for basic waterproofness but they are not very durable. I was looking at some Maxpedition pouches yesterday and wondering if they would work, then I thought about my spare mini-camera case I have. It has a velco belt attachment and is about the right size. I can attach to either my main kit or to my person separately and perhaps use a altoids tin for the inner container.