I'm building a seat pan for a customer, and I think you might like the method. Considering you are going to reupholster your seat pan anyway, my method won't cost much more at all.
1) Cut a piece of wood (1" x 12" x 2') and trim to create the outer profile you want for the seat
2) Weld a pan on the bottom of the rear frame rails to mount electronics and the battery
3) Add additional wood layers in the rear to provide room for a dry-cell battery to lay on its side on the electronics pan and still clear the bottom of the seat
4) Lay your fiberglass over the wooden form and use it as your base instead of the stock pan
As long as you do not raise the rear section too much, you will have enough space for a passenger. The seat could look mostly stock, or could be sculpted as you deem fit. The benefit is that the battery and electronics can fit under a stock-ish looking seat and still clean up your triangle.
I'm considering a battery similar to this:
http://www.trojanbattery.com/Products/documents/RE_ProductSheet_000.pdf It is a dry cell that can be laid on its side, so the profile will easily fit under your seat.
Camelman
