I put together a pretty straight forward box for my car utilizing the spare tire area (I can't fit the donut over my brakes anyways )
The box is 30” x 20” x 9” out side dimensions using ¾ MDF. The interior is divided. I lost about 2 to 3 inches of overall depth but retained a large portion of volume of the trunk.

You have to trim out some of the Styrofoam that makes the false floor. This is pretty easy with a sharp razor.

I was given a set of older JL 12’s and a Kenwood amp so I figured the worst that could happen is I’d need to replace the subs in the box to get some better response and volume.

To my surprise I got great response and so much base I have turn the gain way down and the sub out put way down on my head unit to get them blend.

The box was pretty easy to build -I'll get some pics up in the near future - I used a carbon fiber veneer that didn't give the effect I was hoping for so I'm going to try a turned aluminum veneer next. I'll post the dimensions of each piece later if anyone is interested in an easy to build box. The design could easily be changed to accommodate 10’s or even a shallow speaker to retain the original depth of the trunk.

I think I may pull this box and build a single 10 enclosure to fit in the same area as the 12’s overpower the rest of my system.

Once I have the dimensions for that one I’ll post them up if any one would like to build one for themselves.

I did some searching – and short of having a custom box made I didn’t see any options for enclosures using this space. Are there any out there that I missed?