Perhaps if you showed a picture of your intended location for a fuel tank, we might come up with something. Also, have you thought of just making one to fit it's "address"? That way you can use all the available real estate. I'd just scrounge up some 1/8 plate. Form/bend and weld up a tank then weld in the fittings, any mounting tabs, and filler neck. You could even cut/drill a hole for a fuel gauge if you want. Rinse it out good before you mount it and you are good to go!