[MercuryFlint]

You'll just need to pick a sub that works well in limited space/low enclosure volume. You'll need to measure what mounting depth you're working with. In space-limited enclosures you may have to go with a smaller sub or even a shallow mount one. It's not going to give you crazy big booming bass compared to a bigger enclosure with a big sub, but you can still have a good sounding system.

You'll want to build a box to fit in the space. You may be able to seal the space instead but I'm not familiar with this or the difficulty.

You could get fancy and go IB, but that's a lot of work.

 

[MercuryFlint]

Installer wants to put a JL truck wedge with a 12" TW3 in that space.

Custom box pricing I've gotten for the spare tire space has been insane compared to just using the pre-loaded JL box.

Guaranteed. For a subwoofer box in a custom location you're going pay a pretty penny or you'll have to learn to do it yourself. Trust me, I know, I had to build my own, and it was not a beginner-level box due to space limitations. But not everyone is equipped or interested in doing it.

I've heard good things about the JL loaded boxes, but I can't speak to them myself.

You can look up your vehicle on Crutchfield and do some googling, they may have subwoofer boxes that fit against the side panels in your cargo area, which would give you the storage space you want. Quality can be hit or miss, due to thickness of materials, and may not support a higher power sub, but you may find one that does the job.

 

[MercuryFlint]

If the extent of your audio upgrades is just going to be subs that you may as well just go with the loaded JL box and call it a day. If nothing else you don't want your subs to outmatch the rest of your system, and I think the loaded JL box will probably suit your needs. Your sound will greatly improve.

 

[MercuryFlint]

That actually a lightweight honeycomb corrugated/card material and the surfaces dipped with fiber/resin. Bass/sound will go right through it. I have a picture/cutout of it somewhere on one of the threads.

I was thinking that a fiberglass enclosure wouldn't be that difficult to make for that cut out. I've never heard of using corrugated cardboard, that's a really neat idea.