+1 on Bill's and David's comment. One trick is get good foam and cut it and glue it to make the shape. Design it so you glue on a top layer of "temper foam" or memory foam (invented by NASA for space missions). It is a foam that forms to the person with weight and body heat. You do NOT want a whole seat of it, as it will compress too much and you'll sink. So a layer of it is great on top of good closed cell medium density foam. It will make a nice cushion for your seats. The trick is finding a vendor to sell different foam types and sizes in smaller quantities.
You can keep carving and adding and subtracting (before upholstery) until it is perfect, then cover them. I did that on my first RV, RV-4. It came out great. I had a local upholstery shop cover them. The middle of the seats had horizontal tuffs or "pillows" with some filler to make it look Pro and very comfortable.
Ideally your seat cover material should be fire resistant. Many synthetic materials burn fast and hot. Aircraft approved stuff is flame retardant.