I find that my butt doesn't bother me as much as my lower back after being in the same position for a long time. My simple solution was to take a bath towel, fold it in half length-wise, roll it up and put a couple of large rubber bands around it. Whatever it takes to make a roll that's about 4 inches in diameter.
When my lower back starts hurting, I slide that towel roll behind my back at about belt level. I sit up straighter, and it changes my spine angle. In a couple of hours, I remove it, which changes the angle back again.
I used to have a Ford Explorer that did the same thing by pushing a button that blew up a built in pillow in the lumbar area. My solution is a lot cheaper.