Yup, I'm familiar with the concept -- Never built a box.. but I know subs are rated for X cubic feet of space non ported,.. and a different rating if ported, etc. I'd probably match sub to Box rather than vice versa -- Sounds weird, but money wise.. the enclosures for even ONE sub,.. is WAY more expensive than even a mid-high end sub. (And I'm not doing more than one sub.. I don't want car rattling bass.. I want that clean punch.. that's all.) Which is why doing something in the factory location behind my passenger seat would be super ideal.. cause I don't want to take up room in the trunk,.. and a rear facing sub is going to push out insane bass in that car because of the shape.. It would be rear facing on a hatchback,.. and I know it'd be like the 300ZX .. I'd get A LOT more out of a W1 JL with even 100-150watts on it than some get with 500+watts and better subs. Sounds odd, but the car and where the sub is placed makes a HUGE difference.
Subs with less airspace,.. behind a seat, etc.. Are going to be weaker, by nature.. and for my purpose that's really a "Preferred" thing as odd as that sounds. Which is why something like an 8" JL Audio sub with 75-100 watts RMS would be ALL I wanted .. mounted behind the seat, or in an enclosure in the corner like that one picture I linked of the Wicked Cas enclosure:
Wicked C.A.S.*::*MAGIC BOXES*::*Nissan*::*Nissan 350Z Corner 1x10 Enclosure Sub box Subwoofer enclosure
Check it out.. $370 JUST for that enclosure,.. I'm obviously NOT buying that. Be nice if I could build something like that myself,.. but that's less woodworking and more molding. So again, I think the most ideal place that would fit in my budget is going to be a sub in the slot behind the driver seat. That slot is going to need work to build it up for whatever sub is put in it.. unless I do like a few have done and put an 8" sub ALREADY in a box that fits in that space and just seal it. Though just throwing a boxed 8 in a slot like that then putting a plastic cover over it seems like it'd sound pretty bad. So, somehow mounting an 8" JL sub in that factory slot (you can buy the factory housing and grill etc to make that slot behind my seat ready to put a sub in.. -- ) But since you aren't a sub guy, you probably can't help with that area of expertise. ?