Gas can mount on top of spare tire on back wall of trailer

Well, it's a whole lot better than I do! When I'm bringing the motorcycle (on a hitch mounted motorcycle carrier rack) I bungee cord the gas can to the rail. Not to mention the fact that the bike itself has a 3 gallon plastic tank too!

And yet, somehow I'm still alive...

I'd echo the concern someone had above about the strength of the rear wall, but I assume you'll keep a close eye on that for any signs of impending failure.