Where are you based, Glenn? There are some good " seatbuilders " out there, but you do really need to see them face to face and get a feeling for their abilities. Any couriers in your area? They tend to know people you can trust. We used to use a chap in Essex, he adds foam and a new cover for around £30-50, but it's still touch and go a bit. Off roaders push your weight onto the coccyx ( I think it's called! ) which is the little bone in the very pit of your back. If you are running into agony here...it's gonna be difficult! You need a wider seat, and the frame has its limits.
If you are not in Essex, try to catch a courier! ( If you are, let me know and I'll send you this chap's details )