Pain in the...

[Inhouse Bob]

I got lucky and bought mine with a Corbin already on there, but I understand your reluctance to spend that much. Several folks have had their seats rebuilt by upholsterers and had good results. Basically, they just add or replace the stock foam with something firmer. It might be worth pursuing. On the other hand, several seats have come up on this forum. Just watch the classifies every day because they go fast.
Good luck!