My wife is thinking we will try using some generic foam. We have a two-piece raw foam we used as a mattress in our first rig. The two pieces and it's size has me concerned, but surely worth giving a try.
Back when we bought our PC in 2007, a zippered mattress cover was an undocumented option, made of the same fabric used elsewhere in the PC. We never bought that cover, now kicking myself that we didn't. It would have been a great solution for using generic foam.