Well has anyone ever thought to get a stainless steel bowl and weld it to a plate for a behind the seat tank? You would have to add a filler hole and cap and a gas line outlet. Some thing like this is cheap and would hold a lot of gas.
Amco Houseworks 9 Quart Stainless Steel Mixing Bowl: Kitchen & Dining : Walmart.com
Amco Houseworks 9 Quart Stainless Steel Mixing Bowl: Kitchen & Dining : Walmart.com