You should be able to get some metal Jerry cans (no idea what the currently politically correct term for them is) and mount them - 5 gallons on each side, 10 gallons total, and make sure you get something to pour with, because they're designed to pour into a Jeep, not a motorcycle.
The "fuel box on the passenger seat" is a thing I've seen reasonably often on BMWs, though based on the rest of the bike, I don't think they're cheap. They also serve as a location to put stickers you've purchased from various places. I assume there are random homeless guys camping along various adventure trails selling souvenir stickers or something...
Really, though, there's no great solution. Walmart gas can is actually one of the better options out there, especially if you value "cheap" and good enough for infrequent use.