May be a Butt Lift is on the cards.
Lunges & squats. They'll make more cushion for the pushin'.I've been thinking of getting a boob and butt job but now looking back it might not be a great idea and not because I might regret it but because sierra1 might find me less attractive.
If the big-name seat makers can't help, go to a good local upholstery shop. Tell them what you want and let them work their magic. I know L.A. has at least a thousand of mom & pop places. They'll go in and carve the cushion to where/how you want it. Trim up the outer cover, and it'll look factory. Seen it done with OE seats. Yours shouldn't be any different.