You can get high density seat foam at JoAnn fabrics. That's what I started with and it was in fact more comfy than the camp pad, BUT it takes so much more of that stuff to work that you end up with a really high seat. And it's pretty expensive.
The other thing you might consider is one of those gel saddle pads you see advertised in the sidebar to the left from time to time. Or maybe even take a couple of gel bike seats and arrange them under some upholstery.
In the end, a good seat that is shaped right doesn't need to be super soft, it just needs to distribute pressure all across your ass.
Dan