I'll start here with how I build the box, you already have the dimensions, I used 3/4 MDF, figuring how to mount it was a little tricky but I found a good way to do it without cutting anything, everything I did is fully reversible.
Building a box is incredibly simple once you get the cuts (home depot and lowes will cut wood for free fyi)
start with this first, put your board over the tire mount and smash it with a hammer to get an imprint and figure out where you need to cut it (easiest way for me)
wood glue is your best friend
i picked up this air powered staple gun for 5 bucks, works great, staple all the sides together after you glued, wait until sort of dry but not fully dry as you don't want cracks.
make your cut.
cut your hole out and use a 3" pvc pipe, this has to be somewhere around 2.5"? after put a 3" pvc cap on it and drill a hole in the middle of the cap for your tire mount.
glue, staples, and silicon, makes the box indestructible and make it so that your box is sealer and air won't escape besides the port!
I always use T Nuts for subwoofer boxes, you never know when you'll want a different subwoofer and you don't want to keep drilling holes, the port is on the side, it's the side with the most room possible.
Now this part was a little hard, ideally you need a piece of board 3.5' x 5' to trace out the trunk carpet and make your cuts, but I just have my Z so I improvised, you'll make due without me going into details.
Make your center subwoofer cut first to make sure the subwoofer hole matches and the boards fit snugly!!
this is after I finished carpeting, ideally i'd want the port to be in one of those holes but this will work just fine, each hole is a breather slot, makes it so the subwoofer box won't suffocate and the port can pull in air freely, seeing how it's 11pm at night, I don't have any speaker grill cloth, those holes will get covered tomorrow ASAP.