Sorry for the delay in responses. Have been quite busy putting all of these together.
Changed the design slightly, and interior comes out to around .40-.45, and probably larger since the polyfill fools the woofer into thinking the space is bigger.

Panel can be put back on after installed, but I think it sounds amazing with it off. Some people have cut a grill into the compartment cover, I think I might do that soon as well.

The one pictured is set up for any shallow mount sub really. I think it's a nice happy medium on interior volume. Not too big and not too small. I'm honestly still amazed at what my little 10'' can do. It serves all of my "bass" needs so to speak.
If you need anything more than that, most likely you would have to abort the compartment enclosure, and have a regular box and lose your cargo area. But I think anything more than that is just obnoxious. Even at full blast this is pretty outrageous.
I think the biggest thing that does it for me is the fact that I can actually use my entire trunk. This is the first time I've been able to have the bass, and the space (Might have to name it the "Bass and space package"!)
Also, if you have an 8'' I can just cut the opening smaller, just would need to let me know.
I would recommend the pioneer, however I know that JL just came out with a shallow, but just by the looks of it, it looks pricey....
I'm on the verge of buying another one though myself and maybe making another custom enclosure around the spare tire well.

If anyone has any questions though, please let me know. I'll be putting up pictures shortly.