A comment and a question:
Belly Pod: Wouldn't a belly pod on a PA20 or PA22 interfere with the landing gear structure that is exposed below the belly where it connects with the hydrorbs? A PA18 has different gear with nothing aft of the cabane I think.

Rear seat/cargo area: on my '55 PA22, I separated the rear seatback from the sling, and recovered the back of it with fuax leather. Bought a new sling from Airtex (I rate this item a 4/5, not nearly perfect but will do the job) and added a couple 6" long, self-adhesive 2" wide Velcro strips on the sling and seatback aligned with the upper crossbar to keep the seatback secure.
It takes less than a minute to convert from four seat Chevy coupe mode to El Camino mode.
The sling can become a tonneau cover to keep loose items from reaching the forward cabin area during turbulence or rapid deceleration, or rooled up and stuffed into the hat shelf.