Thanks for all the input on carrying spare gas. Liked the spare tire container best, but a little pricy on eBay, I'm going to keep looking for one though, might get lucky. Decided to install a receiver hitch and fabricate a small carrier. Did get a response on the swelling from Attwood on their plastic tank, "we have designed and test the tanks to withstand the pressures."  Still a little uneasy about it swelling up like a watermelon. Would rather keep it outside behind me, than over my head. 
Roger