First you need to choose your cans. US cans like the Blitz cans are a three price design with a flat bottom. NATO or Jerry cans have a two piece design with a seam down the side. The NATO cans are the ones to get unless you like leaking. Now the problem is mounting. I do not recommend mounting cans to your stock bumper. Two problems with that. One either you have the can mounted to the face of the bumper or you have it stand on top of the bumper. In one case it will eat up your departure angle and having a gas can become a bumper isn't a good idea. If the tank is mounted on top of the bumper it might cover your taillights which isn't good. Not to mention gas cans like to be resting against something and not free floating. Gas cans will need to be RED. Water cans are BLUE. I guess if the cans are new it won't matter if you have water in a red can unless you happen to forget. One other problem you will have is if you plan on opening the tailgate. With a can on the passenger side, it won't open. Really, there is very little room to mount a can back there.
I've seen some make a quick disconnect for the spare tire and mount a can against the spare. This keeps the can high up which is good. Back in the day, it was common for a Jeep to have a can mounted next to teh spare but the spares are a lot larger and there is little room with the tailgate and rear lights. You either have to remove the lights and relocate them or relocate the spare. There are a lot of options if you move the spare to a tire carrier. But that's not a inexpensive option.
