cafe style seat.

You could take a plaster mould off the back of your tank, or a tank you like, and cast it in fiberglass. Have you looked at the shape of the back of a Sportster tank?

Easiest bet? Check Airtech Streamlining for a cafe seat thats wide enough to clear your seat, cut the pan off, notch the rear portion to fit over your sit, drill some holes, and bolt/wing nut the thing onto your bike.