For longer drives, I've used the
Purple Royal seat cushion, and it's extremely comfortable. The seat goes from the least comfortable car seat I've sat in to the most comfortable.
Sadly it's not a permeant fix for a few reasons:
1. Max temp of purple seat cushion is 105F, so you can't leave it in the hot car, otherwise it apparently deforms.
2. Bottom seat heater won't penetrate it. So when it's really cold out, the cushion will be cold and will stay cold for a while.
3. The cushion is thick and raises your viewing heigh a lot, might be an issue for people over 6ft.
So I end up only using the cushion for long drives, but it is wonderful for those. For a more permanent solution, I'm going to be experimenting with extremely stiff foam between the metal springs and seat cushion to raise the middle section, I'll report back how that goes.