I have a 99 X-star and am kinda curious as well. I don't have much storage space left and what I think would be really trick and easy is to sink in another batt right next to the stock location under the passenger seat, fiberglass resin it in, and place a battery box similar to the stock one there.
I'm a bit fearful that I might encounter something more solid under the floor than foam (like a stringer). Anyone have any ideas on how to determine that before cutting up the carpet and floor....short of X-ray or ultrasound???