Just thought I would give an update here for anyone who cares.
I went to a local upholstery place and picked up a 6" square of the hardest, densest stuff they had. Was still softer than I wanted, but thought it was worth trying for only 30 bucks.
Very happy so far. For me, it has taken most of the shock and compression out of a normal box and it's harder to get off than just the box.
If anyone has wanted to try the foam, I would advise going this route. Can't beat it for the money.