You’re going to have to buy high density foam and upholster a pad out of it. I went to JoAnn Fabrics and bought a 2x2ft 2″ piece of foam for $12 and a yard of blue vinyl for $20. I purchased connecting foam floor from target for $20 for the knee pad.
I got my plans from Dani, who got them from Brandon.
Bill is having the tables fabricated to be top notch functionally and aesthetically, so if you want to wait a bit you could just buy one from him.