Mike,
you are quite right. It would not be difficult to do similar for so many different cars.
Mark set out to do the very best quality seat possible, especially as he had to have Gulf approval, but it would be very straightforward to do a "conversion" in not so much fine detail or finish. Certainly would be cheaper and possibly more user friendly without worrying about damaging such an immaculate product (drinks spill, leather tear or scuff, burn mark, cat/dog claw scratch,etc) if you are doing a one off for yourself.
As he wanted to market these seats to a discerning market place, they really did need to be perfect, and with that level of work there has to be a price tag.
Lola T70, GT40, McClaren, etc, etc, would all work well.