Wanted a spare gas jug, but didn't want one hanging off the back of the Jeep, and didn't want fumes inside since I tend to enjoy a cigar while crawling over stuff. Rotopax was the answer. This one is the two gallon unit and price listed included their locking mount.
I cut two 14.5" strips of 2" wide 1/8" steel and bolted the mount to those. I ground off the excess bolt on the back of the mount so it would sit flush to the tailgate (remaining bolt and nut fit into the middle indention in the tailgate). Then predrilled mount holes in the stripes, sprayed it black and attached it to the tailgate with self tapping sheet metal screws. I know, the strips aren't straight, but the tank is. Tailgate opens and closes like normal and if you still have the connection cover on the passenger side of your tailgate, the 2-gallon still fits between that and the latch cover.
Works great, no leaks, no fumes, and it's secure so it shouldn't walk off or bounce around. Great product, but they are pricey!