Brian,
Here is what I was thinking you should do for an enclosure in your car.
The top picture is a side view as it would look from the passenger side of the vehicle. The right side would follow the angle formed by the rear seat. The bottom pic is what it would look like when looking from the rear of the vehicle.
The raised section on the top pic actually fits into the area where the factory hole is. The total depth of the enclosure is about 7.5 inches. But only 4 of those inches will be visible while standing up and looking into the trunk.