Okay, got my measurements...
Max square dimensions facing up are 39" width x 17" height x 16" depth.
If I angle the back of the box to match the back seat will give me 16 top and 23 bottom depth, averaged depth being 19.5"
Max square dimensions facing the rear would be 39 x 17 x 15 depth.
Angled back = 15 top and 22 bottom depth, average is 18.5" depth...
Square box facing up 6.13 net
Square box facing rear 5.76 net
Angled back facing up 7.48 net
Angled back facing rear 7.10 net
Keep in mind, the box is going to be 1.5" thick all the way around... so after the volume needed for the box, I only have:
Square facing up - 3.79
Square facing rear - 3.5
Angled up - 4.81
Angled rear - 4.2
I haven't taken out displacement of the woofer OR the port yet either... so keep that in mind!
Just for reference,
Using the square facing rear dimensions, I could exactly fit a 2 cubic ft box using a 42 sq inch port for 2 XL 10s tuned to 36hz.... or a smaller port tuned lower.
2 15s will not be possible facing any direction.
2 XL 12s will only work if use a tiny port or tune high facing the rear, facing up I could get a 40 sq in port tuned to about 35hz.
3 LVL 4 12s will only work if I face them up, go with a single 3/4 thick box, use a 33.75 sq in port tuned to 34hz.
1 LVL 5 15 will only work if I use a 30 sq in port tuned to 39hz facing up... IMO that port should be at least twice that size if not more and I would want to tune to 32 or so for that sub I would guess...
What port sizes/tuning and power combination has been found to work best with these subs? I guess I should have asked that to begin with.