Yes, this too was/is on my list. I took it a step further and fabricated a skeletal structure to raise the backrest cushion. Great structure, poor result, needs more thought. I raised it about 2" too high, so when in place it's actually less comfortable than originally! And, when I snap it in place at the higher location (it's movable), the snap straps on the rear of the cushion hang to support the cushion rather than supported by the top of the seat as before. So my best guess is to lower it 2" (easy) then fabricate some sort of lengthwise lower edge to support the bottom of the cushion.
Sorry, no pics, wasn't all that proud of the attempt, but I can post some if others don't want to repeat what I did. :sad