Simple, cheapo mount...
I had been looking for a safe and secure way to store extra fuel on my 3rd gen 4runner for a while and finally settled on 2 3-gallon Rotopax cans. That 6 gallons bumped my fuel supply from an inadequate 18.5 gallons to a respectable 24.5.
As for mounting, I had an old Yakima rack and "basket case" roof basket that I bought 20 years ago. All I had to do was find a set of "landing pads" and "control towers" to attach the Yakima load bars to the factory rails.
The cans were mounted flat on the basket, side by side, secured with a ratchet strap that runs through the center hold of the Rotopax and then cinched down tight, with a piece of cardboard to prevent the ratchet strap buckle from wearing thorugh the plastic.
You can see them mounted here, just before I left for Moab:
Here's another shot from the rear.
I don't have any close-up photos available, but it was a pretty simple mount, I just ratcheted them tight and they didn't move. Not only that, they didn't leak even a drop, despite the temperatures that fluctuated from lows in the 40's to highs in the 80s or 90's.
I did find the "California" spouts to be a total PITA. I'm hoping to get simpler spouts that don't have the "auto shut off" feature that these CARB compliant spouts do.
Overall I was very impressed with the Rotopax cans. expensive, yes, but if you need to carry gas safely in rugged conditions, this is the way to go. A "blitz" can would not have fared well under these circumstances, I'm afraid.