Use a frame from a seat at the wreckers.
Just strip it down, make your desired design out hi density foam,
or add low density foam to the existing , until you acheive your desired shape.
Cut the sections out of the finish material you want,
then glue a final layer of foam to the inside of the material,
Then stitch together.
Look at the way the old one is put together as you pull it apart.
Take photos of each process as you do the dismantle.
I'm sure if you get the shape you want and have the material, an upholsterer will do the rest at a considerably less price than buying a race seat.
Hope this is of assistance for you good luck.