Here are the specs that you requested. I am attaching a couple pics of my build process to assist in clarifying the sizing...
View attachment 458719
The box is 15.5" wide across the face.
At the front, it is 12.5" tall. At the rear of the box, it is 11" tall...it has to slant down as the rear speaker tray of the Mach system is angled which resides in the back window.
The box is 20" deep at the bottom as well, and the angle that my box has matches the angle of the rear seat. I did not have my angle measurement tool here as my father and I designed and built the box together, but you will be able to get that measurement off your car.
Lastly, we crafted the box from 7/8 MDF, and it is doweled, screwd, and glued, with a silicone resin caluk on all the seams for an air tight seal.
I just completely re-did my trunk again after getting my Alpine headunit installed, and I removed the spare tire for an upholstered storage area. Also, I highly recommend JL Audio's carpet as it is easy to work with, and looks great.
I hope this helps.
~Darren