As a suggestion, don't know if this can/would work for your set up, but you say that a nalgene bottle could work if you had the space for it. Since you are using a maxpedition pack which should have a plethora of PALS webbing points, is it feasable to attach either a small rolly polly or their new water bottle pouches to your pack? I'm thinking the rolly polly if you wanted the ability to collapse the pouch when not in use, the water bottle pouch if you wanted to squeak a little extra pocket space (they have a zippered pouch on the side). Just an option.

-Bill Liptak