You can also buy mundane, steel, cylindrical tanks from Manchester Tank. I've got a 35-gallon (28 fuel) wedged in the spare tire area of the wagon:
http://wps.com/AMC/1963-Rambler-Classic-550-Cross-Country/new-tank/new-tank-close.jpg http://wps.com/AMC/1963-Rambler-Classic-550-Cross-Country/new-tank/new-tank-detail1.jpg http://wps.com/AMC/1963-Rambler-Classic-550-Cross-Country/new-tank/new-tank-detail2.jpg http://wps.com/AMC/1963-Rambler-Classic-550-Cross-Country/new-tank/new-tank-detail3.jpg http://wps.com/AMC/1963-Rambler-Classic-550-Cross-Country/new-tank/new-tank-detail4.jpg http://wps.com/AMC/1963-Rambler-Classic-550-Cross-Country/new-tank/new-tank-detail5.jpg
It's not obvious from the photos, but the tank area is open to the road underneath (like 10 sq ft), and sealed from the passenger compartment by the hatch and carpet.