I want to see your new rear sofa seat - sounds ingenious if it is more than just a strapped on cushion off the lounge
you will be disappointed when you see my homemade seat it looks unprofessional but it helps my wife generous rear end . basically it is an inverted U
i shove the wings between the panniers and the frame and the middle still rest on the original seat nothing fancy and I pull it off in 2 seconds when we stop for the day and all the rear " stuff" comes off when I ride solo which is 99% of the time