Just stop. I personally hate Camelbaks and the like; when I'm riding hard I overheat if I have my back covered, the weight on my body is not a good thing either. The behind the seat bottle cages work but, depending on the cage used, can eject bottles over bumps. We call them rocket launchers. You can rig up all sorts of solutions to carry more water but why?
FWIW, I regularly ride 70-80 mile rides in Florida heat with two bottles. Anything longer, I stop to refill.