After installing your access panel, you can probably add some pvc cleats and remount the back-rest pad seen in your picture, cleats would give you the additional clearance needed due to depth of new panel...and depending on size of access panel, it would remain hidden behind the pad, just a thought.
I replaced my 8" transom access hole with a nice 13" x 23"
TEMPEST access hatch, HUGE difference in getting in there to do stuff. The door snaps right off the hinge so it's out of the way when needed....My CC has its access door on the side, it's a bittch getting in there to do anything, I feel like a fat anaconda with arthritis when I jockey into position.......feel like I fell off a building when trying to get out.
Good luck.... research and patience are key