Rebuilding a automobile seat

I spend many hours each day in my car and the seat is very uncomfortable.  I have a friend that will help me pull the cover off the seat and rebuild it to fit my back better. From reading Esther's book, I feel that it should have a recess for my tail to fit into and the rest of the seat back should be pretty flat.  I am think of adding a 6" wide band of foam up the middle of the seat to support my spine and allow my shoulders to go back.  Am I on the right track?  Any advice or reference would be much appreciated.  I would like to get it right the first time if possible. Thank you.