I think Steve was talking about the O&N Auxiliary Fuel Tank [onaircraft.com]. It's a 14+ gallon tank, shaped to fit on the floor of the baggage compartment with an external fill-cap. Pretty slick, and FAA-approved.

-Kirk Wennerstrom
President, Cessna 150-152 Fly-In Foundation
1976 Cessna Cardinal RG N7556V
Hangar D1, Bridgeport, CT KBDR