Replacing Front Seat Foam

The foam in the front seats of my Ford E350 Class C is continually decaying to the point that the seat bottom's provide little support. The fabric is in great shape. I know you can buy sculptured replacement seat foam, or have a reupholstery shop redo the foam, but in doing some research on the subject I found numerous references and videos from people who had refoamed their motorcycles seats by building up the foam and essentially carving it to shape using what looked like a sharp bread knife. I was wondering if anyone had done something similar to their RV, car or truck seats and if they could share some information on the subject.
Thanks,