Next you want to set the new cut angle of the box up on to the table saw and set the blade to the same angle:
Rip another piece of MDF that is large enough to become the new angled end piece, with the newly set angle to match the front edge. Then place the box on the MDF with the bevel cut flush with the front edges of the box, and trace around the top, bottom, and back:
Cut that out with your choice of saw and glue & nail/screw it to the angled end of the box. It should look like this:
Once it is dry, you can shape the other three edges of the new end to they are flush with the back, top, and bottom. Next apply a bead of caulk to all joints for extra assurance that the box will be sealed. (as seen in the above picture)
Cut out the front face, but take note that the speaker hole is sized for the Rockford Fosgate sub and you may need to adjust it if you choose something else. You can adjust the hole up a bit and to the left quite a bit, but not really much further to the right and not down at all. I actually needed to sand out a small divot for the magnet to clear on the bottom piece.
With the face and the box still separate, now is the time to test fit it in the car. It should fit just fine, but if not now is the time to find out and make adjustments. If all is good, mark the location of the face on the box and remove both pieces.
Glue and nail/screw the face to the box using the marks you made while in the car and let it dry. Caulk all joints for the face on the inside and outside of the box.
I didn't take pictures, but I installed one of these in the top of the left end piece. I believe I used a 2" hole saw, or maybe 2-1/8".
I found a black rug at Menards that was something like 6x8 feet for $15 and a piece of it worked perfectly for covering the face. I didn't cover any other sides. Before cutting the carpet, put the box back into the car and make note of any places (like the bottom edge) that you want to run the carpet long. Once you have done that, glue a piece of the carpet on with 3M spray glue. Cut out the hole. Don't install the driver yet.
I attached the box to the floor by drilling a hole through the angled part of the floorboard and using a 4" bolt:
You need to take off the front belly pan to get to this area. You want to drill from the bottom, up into the footwell and not from the inside of the car outward. You risk hitting the coolant pipe that way. Put a nut on the bolt on the inside of the box with some thread locker so it doesn't come loose.
Install the driver, piss off the neighbors.
