The humble stem pouch. Once you start using one, you'll never go back. I think you'll like this one from Blue Lug. They are on the large side. They'll hold a Nalgene bottle without so much as a whimper. 3 velcro straps to keep everything in place, and because it's Blue Lug there are some color combinations that you don't usually see from other makers.
These bags don't have the structure that I'm used to seeing from other brands, but I'm not worried. The folks at Blue Lug do their homework.
