I'm getting ready for a tour of Big Bend Park in S. Texas. Water is a definite concern.

I have 4 btl cages on the bent, but feel I need 2-4 liters more. Will be no room left in panniers for extra bottles.

I'm looking for a bladder that will fit/hang behind the webbed seat of my bent, in a space 2" deep and about 14" wide. And I don't want to pay more than about $10-15 for it, as it'll be a one time use.

Anybody got any ideas/experience for/with an inexpensive, reliable bladder? A DIY?