When my Dad had a pick-up with a topper, he went to the junkyard and got a whole back seat from a car and installed it in the bed, the back of the seat went against the back of the cab, and the seat essentially filled that whole "wall" below the window. That gave a cushion against people bouncing against the wall of the cab on sudden brakings (we'd just hit the seat, as in a car) and gave us a nice place to sit, and seat belts. It was bolted to the bed of the truck, and could be removed if he needed the cargo room, though I don't recall that ever happening.