My boat came with an uninstalled plastic rectangular tank. It fits between the two vertical pieces of ply that supported the round one, and is contained & supported by additional plywood, as well as, metal strapping. It *may* be a Tempoproducts under-deck / Tempo TP18T, I cannot recall the model # or dimensions. Havent used it yet, as I still have to get the engine aligned, and test run.
Also, while you are messing with fuel lines, and such, check the bowl of the fuel filter, I attempted to scrape out the rust, and went all the way thru it!!
best of luck,