Aftermarket seat cushion?

I've had a few as a trucker. The latest (love this name) is the Wondergel Supercushion. Bought at T/A truckstop. Tall, thick, it cuts the vibrations almost completely. Makes one aware of what is transmitted through feet and hands as they are now isolated. I run about 3k miles/wk and often on long stretches of unpaved lease roads out near Mexican border to drilling rigs. That, or very long days on Interstate gives a good cushion cushion the proper test.

Oregon Aero is a company with a product to investigate (posture more than cushioning) and another wheelchair product is ROHO.

Good luck