I've been working on trying to improve seat comfort for some time. I tried installing a foam excercise mat that I cut to size. It provided a slight improvement on my frequent 11 hour drives from NM to TX, but not fantastic.
I was in Target two weeks ago and found a Full size memory foam mattress topper on clearance for $12. 99. It comes in a rolled plastic cover, like a sleeping bag. I cut it to size after doubling it over, and installed it under my after-market seat cover. I just got back from another 11 hour drive, and the improvement is amazing. No more back pain or TB, and every time I sit on it I wish I had discovered it when I first bought the truck. One of these days I'll install it under the factory seat cover, but for now, this is the best cure I've found. I bought one here in NM, and another one at a TX Target store for my 5th wheels crappy mattress, so I suspect most Targets will have them until sold out. I also stuck a piece under the seat back seat cover, too. The only small downside is that I sit higher in the truck, so I had to hit the power seat adjustment to lower it. Well worth the money.