Look at the Graupner fuel tanks at Hobby Lobby. They make some large tanks that fit in smaller areas. Their part #GR238 is 24.5 oz. and is 5 3/4" long by 3 3/8" wide and 2 5/8" high. It sells for $15.90. Hobby Lobby refers to them as 4 stroke tanks.
I don't know how much power is required to tow a sailplane, but I had an Enya .80 4 stroke in my Senior Telemaster and it had a plenty of extra power. A Saito .180.....[sm=eek.gif]
You might be able to tow a Buick with that combo !!!!!