You seem to have plenty of room on the sides / four corners for 20L MWCs. Would be easy enough to fit a spigot to one to use it as a dispenser or wash station.
Too, there are several types of soft-sided water bladders available in various capacities. You could fit a lid on one of the rear side locations and use such a bladder to fill almost every cubic inch of that space with water. A cubic foot = 7.5gal. So if you have say 10"x10"x20" in that rear area, that would give you a little over 8-1/2 gals of water in that cubic. The 5gal collapsing bags are ~$12 last I looked, you could use two, partially filled, so you've got some redundancy in case of a leak.
But very sturdy milspec 5gal / 20L MWCs can be had from LCI for about $25ea. They are far sturdier than the squat blue aquatainers and can take a lot more abuse.