Looking for second, third, fourth and so on opinions.

I have a Victory 4x4 rear bumper with the adventure carriers. I also have a pair of rotopax 2 gallon cans. The cans fit perfectly behind the carriers and rest on the bumper (not suspended at all). There is the tiniest bit of rub between the can and the body.



Here is where I'm looking for opinions.
If I stick some thin rubber in there to stop metal on plastic friction, the only concern I'm left with is gas expanding the can itself. How much should I anticipate these things ballooning? The last thing I want to do is create a bomb behind my hatch, but lord these things fit perfectly.