also i want to commend you for doing something so nobel , and wish you the best with this build
unless we are also going to reinvent the cushion a standardized base to accept existing cushions is the answer. These aren't "everyday" chairs, and most chair users have a prescription pad with their "everyday" chair that they could just transfer from one chair to the other. These pads are $500, but lets get the chair built first! Most common base is 16x16, i'm 18x20, width x length
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It's "Dr. _paralyzed_" actually. Not like with a PhD, but Doctor like in Dr. Pepper.