Get the largest tank that will fit comfortably with a 1/4 inch gap around it (I think it was a 12oz on my LT-40) then pad on all sides, top and bottom with foam. Finally glue in a light ply back brace behind the foam-padded tank and between the fuselage sides. As Michael said, you don't want a heavy fuel tank sliding back in your fuselage during flight.

Cheers,
Neil.