Chris,
I have a question for you. Are you just copying the seat in brass to get the thickness down. If so you could copy the seat with masking tape to get a template. This would help guide you on getting the compound curves by cutting darts in the brass and the push them into position and then solder them.
Hope this helps. By the way would you post what you end of for a template for the brass. It might help others that want to do a seat.
bobthehobbyguy