Try a high end seat out of say a Scania or Volvo HGV (not the basic spec one mind you) you'd need to find a specialist HGV breaker but, IMO, you'll find a seat out of something designed to be driven all day, every day better than the vast majority of car seats where very long trips are generally the exception rather than the norm.
With an HGV seat though you will need an air supply, high end HGV seats have air suspension, quite literally.
Comfy as my old 7 series was for example, it wasn't a patch ,seating comfort wise, compared to a high spec Scania, or for that matter Volvo, DAF, Merc et al.