My car came with an amp and upgraded speakers. There is a subwoofer box that is unsecured in the cargo area, that takes up too much space.

Mtld83 on reddit is convincing me to go for a JBL Stadium BassHub. YouTube video showing BassHub innards, showing a downward firing configuration. More reddit comments. 2addicts.com thread.

The volume in the spare tire can be slightly larger than under a single seat. Also, it's easier to clean under the seat without a box. Also, no impact to foot space. Tools that go in the spare tire volume can be placed elsewhere under the cargo space floor.

However, my spare rim (donut) is 4“ wide and is dished outwards so the inside height is maybe 2”. The JBL Stadium BassHub height is 6.5“.

I could get an underseat subwoofer, but I'm still interested in having a round spare tire subwoofer even if I place it under the seat. I have a inkling that a box shape isn't optimal for a speaker enclosure.

The price for a new JBL Stadium BassHub has dropped as low as $300. A cheaper alternative that isn't as tall: a Bose subwoofer that comes preinstalled on a variety of vehicles. Available used on ebay for as low as $40.

These Bose subwoofers are actually pretty good. See How loud is the Bose subwoofer? Dakoustics on YouTube. The JBL Stadium BassHub is 600W RMS, so it isn't a fair comparison to the 80W Bose. An 80W subwoofer is better than no subwoofer, and the 120dB results from Dakoustics is more than enough:

OEM Bose Subwoofer dimensions:

Dakoustics tested sub is a Bose “GAP4 66 960” for the Mazda vehicles like Mazda3 and CX3.

Well, these aren't going to fit under my seat, and if they do, they'll stick out the back. Back to the drawing board.

I found a BMW X3 underseat subwoofer on ebay for $25 shipped.

Stuffing the Enclosure