My gas Rotopax has struggled in the past. I live around 5500' and it's easy to double that elevation nearby. When filled with gas, sitting in the sun, the Rotopax swells up. It's a pain to manually bleed the pressure off and sometimes it leaks afterward.
Long term plan is to mount them on tire carrier as such:
I originally was going to drill a hole and then plug it with rubber stopper that would allow pressure to bleed off. I was a little wary of drilling that large of a hole though. Thus, I decided on a much smaller hole. So small, no rubber stopper would be needed.
I started by testing the theory with water on scrap plastic.
Took a needle and heated it up
Then pushed needle through plastic
Put a little water on top, no leakage
Gasoline has a lower surface tension than water, but I'm hoping it'll still not leak even if inverted. At the very least, it'd be a very, very slow leak.
Thus, I repeated the procedure on the Rotopax. I put the hole as high up as possible. When mounted, there shouldn't be any gasoline on it and the vapor should be able to bleed off.
I don't have the mount yet for tire carrier, but I'll see if I can't try out the design sometime, driving to higher elevation.