The spare tire area is pretty small. I was able to put a slim 10" that was self contained with a built in amp in the box. I wanted it behind a panel I had made. It's slightly muffled. But it still sounds great.
I cant post pics for some reason. Send my your email or call. and I can send you some pics.
I don't know if you have ever worked with fiberglass but it's not that bad.
The biggest thing with subwoofers is for them to perform properly they need to be in the proper sized enclosure and in a properly made enclosure (sealed, ported, etc.). Think of it like sizing a carb. Just because a dominator will fit doesn't mean it will work better than a smaller carb on the proper engine.
Using water to fill the enclosure to check for the air space volume and to check for any leaks:
Masking off the area and then adding walls to built the fiberglass off of it:
Another one I was building in a GLK350 out of a BUNCH of 3/4" birch in a trunk spare tire well:
Area I had to work with:
Finished off cover with flattened waffle grills and holes cut into the cover board and then covered with acoustical carpet (unbacked that you can see through):
A lot of this stuff required imagination with sitting and looking at things sometimes to come up with the idea's and a plan.