If anyone is interested, I came up with a plan.
3/4" plywood, 48"x43.5", fits perfectly behind the 2nd row seat.
I cut 2 relief slots in the plywood ,2.5"x20", to clear the raised seat rails.
1/4" plywood without the slots, glued on top of the 3/4", gives me a full solid/flat surface.
Looking...