I carry mine of the roof rack, I use a siphon to fill the tank without takin the NATO cans off the roof. I made up a siphon using a outboard motor gas tank hose(has the little bulb you pump) No idea on a rear bumper mount....maybe something you slide into the reviever hitch? but it's either gonna stick out too far or be in the way of droping the lower tailgate.
I also carry mine on the roof rack. If it's for extra capacity for the vehicle you could consider an aux. tank or the larger 38 gallon main tank. As a coincidence I just happen to be selling my 38 gallon main tank. :
I know there are conflicting opinions about the OEM auxiliary tank, but 50L is 50L... and the tank fits snugly into that space above the spare tire crank cross member - and is protected by it, whether the tire is there or not.
If you still want to carry 80L up top, fine... the more the better when you're in the boonies. Me, I like knowing I've got an extra 50L down below and that I'm not likely to hole the tank - always a concern with the over size aux and main tanks.
OK, I've been reading in Tom Sheppard's Vehicle Dependent Expedition Guide about internal storage of fuel cans. Basically, the idea is to use good quality NATO/Wedco cans and mount them in the middle, right behind the driver/passenger seats with the middle row seats removed for proper weight distribution.
I'm thinking about mounting the water cans closest to the outside, to provide a "crush" zone in case of a t-bone, with the gas cans mounted interior of those, with the appropriate ratchet strap tie downs.
My buddy carried gas in his Rover while we were going cross country. I didn't realise it because the windows were down the first day. We slept in the truck the first night and with the exception of one hell of a headache the next morning and a persistant twitch til this day, it worked out rather well .
I had a Ranger in Haines, AK really give me the business about the cans on the rack. I guess the guy was a fireman or rescue person and he said he had seen many times someone roll over and be burned when the cans caught fire. It is fairly common in AK or people traveling there through Canada to carry some spare gas this way. He had a very valid point and I was well aware of the risk when I built the rack. I would; however never have them inside because I have never come accross a can that didn't leak at least fumes when the temp goes up.