Right now I have a single 15" (zcon w/ warden recone) and a taramps dsp3k with a single 6" aeroport 3.8cf^3 @35hz after displacements. It's not enough spl for my liking but I can't make the box any bigger and keep the wife happy with trunk space.
I purchased two sa-12's and was thinking of building a similar spec box. I've read around that ported forward facing is more Sq than spl and you heat distortion more. I won't have to push my amp as hard since the sa-12's won't take as much as my current sub (I think) so I shouldn't worry as much about clipping my amp
My rear seats don't fold down so the subs will be facing the the rear seat backs but with my 6" aeroport centered with my skipass with the box sealed off.
I like the rear firing box idea but I hate worrying about my sub being damaged and if I build a forward facing box, it will be angled towards the rear seat to avoid wasting space.
I only have a few inches of space around the box for the bass to travel into the cabin since my box is tall/wide
I just want the most spl with this set up. That's why I went with more cone area