Starting this now because I've finally got the parts set in stone (no more negotiation), and I'm ready to do some building.
The car: a 2009 Honda Fit.
The sub amp: JBL 1200.1 (purchased)
The sub: Incriminator Audio Death Row 15
The front amp: Profile AP600 (purchased)
The components: Alpine Type X ref 6.5
The HU: Clarion DXZ785USB
With 0.16cu of sub displacement, the end result will be almost EXACTLY 3.25cu tuned to 32Hz with 52" of port area, almost exactly what Incriminator recommends. Although it's rear-firing according to the picture there, both sub and vent will in fact be up-firing. The 17.5" baffle should give just enough clearance to mount the sub without screwing into the top/bottom plates.
This weekend, because I'm bored, I'm going to be building the box. To give an idea of what this box will look like in the Fit, I present my extremely un-scientific way of determining dimensions: a cardboard box.
I found a box in my house that was just under 17.5 tall and 14 deep, and put it in the trunk against my seats. The result was almost six inches of clearance in front of the box, meaning that with a box that's to be 17" tall and 17.5" deep I've got plenty of space. Might even mount the amps there. Here's what I've got as far as trunk clearance front to back (again, that's 14" deep on the box):
And to give an idea of the height:
So there's plenty of airspace, and that bastard's gonna be practically under the person in the rear passenger seat's head.
It'll be a while before the sub itself is purchased so this log might go dormant in the meanwhile, but I'm starting it now just to affirm that, yes, everything's decided and this is gonna happen. I've been browsing the Fit forums and I'm... just about positive I'm going to have the loudest Honda Fit in America.