Upon further research,
@ADVFamilyHauler is right. The military scepter plastic tanks are the best. The US Army appears to have switched to these plastic Scepter military jerry cans. I’m not sure why military plastic jerry cans are better than metal ones, but I can only guess.
So, I’m back to square one with how to carry fuel off the Gobi ladder.
One option is just to go with the rotopax ladder brackets, rotopax ladder attachments and rotopax fuel cans. This option is spendy and only lets me carry two tall 4 gallon rotopax containers. Maybe close to $500 for this option not including the $250 for the Gobi ladder.
Another option is to get two military Specter fuel jerry cans 5 galllons each and then figure out how to mount them on the ladder. I haven’t identified a good bracket to attach to the ladder. Gas containers close to $200 for two and suitable brackets may need to be custom made, so price is unknown.