So if you raise/remove the storage platform in the rear of the Tig, you're presented with the spare tire, surrounded by rigid foam.
I removed the rigid foam, and shifted the spare tire to the right side of the storage compartment.
This presented enough space to custom-build an enclosure for a 10" JL Audio subwoofer (I believe it was the 10W1v2-4).
I chose a Class-D amplifier (again, JL Audio, this time the XD300/1, I think it was), that would fit into the right storage compartment (as you're looking in the rear of the Tig).
Again, pics to come soon.