Owen Bell has underwing tanks mounted to his civilian P337 that he's flying daily. The wings were modified to accept the O-2 hard points, and the tanks have transfer pumps to pump fuel up to the mains for burning, or overboard in case you need to get below landing weight in an emergency. His current tanks are 25 gallons each, and he can make them up to 50 gallons each.
He also has TSIO 550s on the plane that are capable of 350 hp each. If they are running full power, the extra gas is useful.
I'll try to post a photo. You can contact Owen through his website:
www.aviationenterprises.us