Such a controversial topic;
If you do not have an expensive big back bumper system for allowing a spare tire mount with mounts for just one, or better for two cans.., then you are somewhat limited to either carrying the can in the interior, or mounted to a basket type roof rack, i.e., not the OEM style, but the expensive type. Laying a Jerry Can on it's side is doable, but I'd never do it unless an extreme emergency as they can leak, either from a bad seal, or from pressures under the sun forcing fuel to exit, blah, blah. Upright can on the roof is possible if you Jerry Rig the Jerry Can, but that's all up to your creativity. Our roof rack rails are just too low for me to consider. Some sort of 'basket' seems to work best, but the ones I've seen cost nearly a grand with the shipping charges, etc.
Some people carry a stock, (DD), spare tire to the OEM interior spare mount. However with larger diameter wheel/tire(s), that location can work with work-a-rounds, i.e., ropes/straps/ genius use of bits of metals/plastics, etc.., but aside from that, one can just lay the wheel/tire down flat. I do sometimes. When I do; I can place a small plastic red 2 gallon fuel container, (no way a Jerry Can), in it, or wedged between a couple of tool boxes, etc. In the event of a collision I certainly do not want a container of fuel in the interior space to 'fly' around. I consider this spare fuel to be very dangerous, although I have it fairly well secured in that the only way I can see a major issue is if I end up upside down.
One other option, for the littler lower red fuel containers, is one can place the wheel/tire spare secured up on the rear most roof-rack area, thereby placing the red container into the deep 'well' interior of the wheel/tire, and running some aspect of the wheel/tire mounting/securing straps/ropes, etc., thereby keeping that extra fuel as about as secure as possible, imho. I think only one-two gallon red container will fit deeply though. This is my preferred method for travels on the busy highways. Besides that wheel/tire/extra fuel takes up a lot of room in the cargo area where I will need all the cu.ft. space I can get, etc.
If you live in a rural area, you can probably be very safe carrying fuel on the back-side rear of the vehicle, (as per the aforementioned).., but if you mostly consider being in crazy higher traffic areas where rear-end collisions often take place.., think "Molotov Cocktail" waiting to happen. Someone taking well aimed shots at red targets might be an issue. (I've always sprayed paint hiding the red).
Many have added Jerry Can specific-to-fit holders/platforms/small baskets, and have mounted them along the rear sides, above, and behind the rear wheel areas. They are often painted red from the factory. However depending on the terrain you expect to manage, that is not too well positioned if you have to squeeze into, and through tight spots, i.e., trees, boulders, etc., all the while perhaps bouncing around. Not sure if I'd ever mount such contraptions to my lift-tail-gate, either, (requiring interior steel plates to sandwich the mounting system, blah, blah). Again think; Molotov, but somewhat manageable, i.e., by taking the cans out of the holders in order to lift the gate.
I have been wanting to make my own basket-style roof rack that I can add to the existing roof rack in which I can carry Jerry Cans, two spares, and camping stuff, etc. Should be easy with angle-iron, a drill, some bolts/washers/nuts, etc., straps, etc., and pad material/blocks etc., in order to really bulk up. However I have recently purchased a, (small 4'x8', with 16" side-walls all the way around), utility trailer for a wheeler, and any of the aforementioned stuff too. Four, or five Jerry Cans, (in that mix), will certainly get me back to 'civilization' in which to tank-up prior to returning to base.
Hope the above ideas/concerns are universal enough. Some can custom fab-up a larger XJ fuel tank, and others may know of some other fuel tanks that are larger in order to fit our XJ's.., but I don't consider it. Someone else may have additional ideas that you can ponder.., but I'm out of here.
Have fun.