2mm ply rear interior panels covered with vinyl - spray adhesive and fabric glue
This is a little diversion, making the rear kick panels out of thin plywood and the same vinal material as the seat covers.
I got one seat assembled and in the car for a trial. Old seat, new seat, tan seat, black seat.
The gearshift boot is made from the same material with a PVC shower drain fitting as the coupler/base.
The steering wheel is wrapped in bicycle tape and then real leather (for bigger hands to grasp with a bit more comfort).
The plywood base of the armrest is screwed to the carpet from underneath using 1/2 washer-headed self tapping screws before the carpet was glued in place on the rear tunnel.
The speaker assy is just the way I decided to add speakers to the car - easily reversible. Someday I'll make a side-curtain stowage bag, and then I'll have to move them to the doors or something.
I have to find or make some cockpit rails next. I have already made the front one, but might bite the bullet and buy the aluminum shaped corner pieces vs. making them - though I am thinking of forming them out of heated PVC "lumber" (which is akin to steam bending if you boil it), or making them out of 1/8" laminations epoxied together. Too tedious to have to just carve four of them.