Have you considered a small motorcycle tank? Look for kid's dirt bikes or something like an 80cc Yamaha. FleaBay, or a local auto salvage yard that has a motorcycle section might be an option.
There are also new small motorcycle tanks available but you'll have to loosen the purse strings for some of them. $120 to $150 for a small chopper/bobber style. But like they say: "If you're gonna have a party, you've got to pay the band"