The best way (and only Safe way) that I know of to increase your fuel capacity on your sport quad is go to the local bike shop and order what is called the "Oversized Tank" installed by endurance racers to allow them to complete the 80mi race without refueling their tanks.

Some after market sites may still be trying to sell the "Piggy Back" tank but that is like strapping a Molitov Cocktail between your legs instead of the little safer Molitov Cocktail strapped behind your A****.

Be ready to spend some coin for a racing tank though they wanted close to $300.00 for the racing tank for my Suzuki 450, after I caught my breath I kept my factory tank it has taken me 40+mi (one way hard riding)with about an 1/8th of a tank left when I fueled at the half way mark (we had two UTV's chasers hauling fuel for the dirt bikes and sport quads)

Hope this helps