Originally I was going to make a foam mock-up of the seat as in CAD. However, I thought it would be a bit more accurate as an RP mock-up that matched the CAD model as a hard reference for the upholsterers foam. There was a strange 30mm correction to make. Which leads me to think that I have an error in the CAD model, will have to investigate that. But all the other dimensions were close enough.
The seat has had a slight redesign so that it flows into the body panel slightly better. It is now slightly thicker. Up side is that it should serve to cushion my bum a bit better now.