I have a shallow Jl Audio 10", in my spare wheel. Sealed the wheel with a 6" rubber gasket and a 6" aluminm plate over it. Tightened with a bolt(placed a wrench in the Styrofoam). Placed an MDF over the wheel, installed the sub in it and strapped it down with two ratcheting straps. Sounds awesome. I'm using a Polk Audio PA880($40 broken from Ebay, and I have repaired it, for like $7).
It's not nice, but you can see it, under the floor cover.
https://www.mazdas247.com/forum/showthread.php?123848260-Custom-Box-10-quot-JL-Audio-Sub