To be honest, the spare wheel well isn't big enough to get a good sound from
That's not necessarily true - it all depends on what driver you're using.
Some JL subs, for example, have an ideal enclosure size of less than 0.8 cubic feet.
The enclosure I have in my car (in which my JL sub sits) would take up approximately the same volume as the spare wheel well and it sounds awesome. (It's a JL designed enclosure).
Think about some of the 'pre-loaded' enclosures you can buy off the shelf - they're tiny, yet they can still sound great.
What I do agree with is that if you use a sub with a required enclosure size of, say, 4 cubic feet and you stuff it in the spare wheel well it
will sound pap.
So, if you're hell bent on using this space, take some dims of the wheel well, calculate your available cubic space and buy a driver that will work on this space (most drivers will state the thiele parameters on the box, which will allow you to calculate the amount of space required/ideal space).