I saw one aux tank that sits in the passenger seat, it held close to 5 gallons.
Hey! Now There's an idea! A life-size hollow plastic fuel tank in the shape of a heart-stopping 22-year old well endowed female!! Take the helmet off and open the filler cap at the top of the head. Left leg feeds the engine, right leg is the reserve tank! Get snap-on biker chick clothing, some chaps to keep her cool, and a cool pair of sunglasses, and IBA, here we come!