I'll rave again about the Nathan packs/vests. There are purses and purses, so I don't know if the Synergy pack is big enough for your needs - but it would have room for mine, especially if I didn't fill the water bladder completely.

Before I got mine, I thought I'd never use the double compartment bladder - but it's perfect for hot days, when I can put a strong solution of Endurolytes in the small compartment and lots of ice in the big compartment, and not have to worry about the dilution changing as the ice melts.

Two liters of water in my Intensity pack rides lighter than 24 oz in my Amphipod three-bottle belt.