For years now I have used a rolled up towel- hand towel size, roll up tightly the long way, secure the ends with rubber bands - I place it across my lower back, very low, which helps a great deal when I drive. I tried all the seat cushions, back cushions, you name it and they didn't work. My truck seat has adjustable lumbar, and that doesn't help. The simple rolled towel does the trick and you can adjust its location every so often to alleviate any pain. I think a PT gave me that idea. I stop every two hours so the dog and I can get out and stretch, which also is important. Good luck, I know how miserable back issues can be.