This would look pretty nice........I'm not really into customizing my own cars, but me and my brothers rebuild 1968-1972 Chevelle's.......approximately 5 or 6 of them a year, so I understand the attitude and pride of a build-job-done-well and the satisfaction of knowing you've created something unique. I personally wouldn't use the rear hatch door cavity as a subwoofer enclosure for a few reasons:
,) The direction that the subs sound wave travels........point a sub box sub forward, the sound wave travels forward. Yeah, you got bass, but face that sub box backwards and you instantly have more intense bass.
The weight of the subs and the material used to reinforce the door cavity may be too much added weight for the factory door lift shocks to hold the door in it's raised position. Simple fix......
Just add stronger door lift shocks.......but those stronger door lift shocks may (over time) fatigue the door metal where the fasteners are attached resulting in 'ripped' sheet metal where the most stress (fatigue) is created.
Connecting the subs.........how are you gonna cleanly run the subs wires through the factory wire harness loom? Sounds simple to do.... just poke it through and connect it, but there are always blind obstacles and impossible curves/angles which the wires gotta get around.
What I would do....if I were you and sound quality is more important than appearance, I would remove the spare tire and foam storage box, then construct a fiberglass bottom (because the bottom of the xB is an odd shape, then cut a wood front for the speaker box, then a wood back for the speaker box, and finally, wood sides and a wood top. Basically, create a shallow box which looks like the spare tire/storage box floor......Mount one small 8 inch L7 square sub on each side.....your original floor level will almost ne the same, but you'd have no spare tire or storage box......not a prob if you only drive your box intown........get a flat tire, call Triple A for a fix or your nearest buddy to bring you the spare.