I use the Condor Hydration Carrier II Since it has molle system, I attach it directly on the back of my vest
http://www.condoroutdoor.com/hcb2_hy..._carrier2.aspx
Even with my 100 Oz Blackhawk Torrent Extreme II in it there is still room in the large pocket. I use the 2 small pochets for stull like BBs, Batteries, Ration...