I made a new piece to go behind the rear seat, its in the attic now as the car is in teardown (going to the jig next week) Same shape as the piece of cardboard behind the seats. I have a sony xm-250x amp on the left side of the ski hatch then over the ski hatch I made a plate that can slide which has an infinity 10" facing through the hole. Speaker cables run up along where the trunk springs are.
If i need to use the ski hatch i just slide the sub across out of the way. The whole set up is covered by a curtain of black fabrick so when the trunk is open its not visible, and the car is practicle too. I previously had two rockford fosgate 8" and the infinity 10" running off a different amp (an infinity ra6006 in the trunk but the sound was lost behind the seat and the cabin sounded flat. This setup even though the sub is in free air is much more alive.
Its all 12mm marine ply, looks a bit rough but i hope the description gives you the idea!
Rich