Does anyone know where I can get a piece to fill the area above the seat.

I've got a waterproof piece in there below seat level but I want the bit that you see on gp bikes to stop you sliding back too far to finish this area off.
I have to screw or glue it on from the outside as my undertray is fitted before the cowling,I have a ally suport on the inside. Needs to be 50mm thick to achieve the look I want (2 inch)