In my case it's for two .3cubic foot sealed poly filled enclosures in a 4-way system with a woofer, the subs will coupled in parallel and driven by the JL HD900 500w rms subwoofer output.
Here's how it would look like with a DD, and yeah, no space for anything but 6.5" woofers anywhere. Trunk has a solid metal plate in the way too.
Some contenders (right now I'm leaning towards the CDT M6+)
JL AUDIO 6W3V3-4 $100
DLS Reference RW6 $190
Tang Band W6-1139SI $130
DD sw6.5 (hard to find, but might be the hardest hitting option?) x$
Kicker CVT65 $60 (my current woofers)
Forgot this one:
CDT M6+ $150 (12mm xmax, perfect for sealed enclosures, a winner?)
And this one
Rainbow Vanadium $329