First of all if you raise the seat will it interfer with the handlebars when turning? Just something to check before going any further. If I were to raise my seat I would want something light, something that would not soak up water and ,most importantly, something that will not rub through the tunnel or seat bottom. High density styrofoam comes to my mind. It comes in different thicknesses, light and easy to work with. But if you put something between the seat and tunnel, how are you to attach the front of the seat to its clips? Don't think you can raise the fuel tank any either as it is molded around the steering stem and steering brace. Sounds like something that can be done..... as a summer project.