This is so simple, I'm almost embarrassed to mention it. For my water filters (I have a ULA and a Kataydin), I have adapters that fit wide-mouth Nalgenes and small bottles, but nothing to fit a Gatorade or PowerAde bottle, and these are my current favorites for carrying water. So I took a cap from one of the bottles (I have quite a collection) and reamed a hole in the top using a skinny blade from my Swiss Army knife. I enlarged it until the filter hose just fit through, and that's it. Cheap, replaceable, and functional.