I've done that before. no 2 sides are parallel. It's a bitch to figure
out. especially when it has to be exact as when building ported
enclosures. What I did was break the box down into the various triangles
and figure the volume of each one.

yupz, a trapeziod. To be exact, it's actually a trapezoid on top of a
rectangle.
Due the 45-75 degree slope, I guess the subs can only be mounted by
increasing the height. They will be firing at the rear at such an angle so
that the sound will be reflex to the front of the vehicle.
It's a Dual 15" box with slot port that I wanted. I have no restrictions on
the box size.
any suggestions?

subs specs
Fs = 28 Hz
Qms = 4.68
Vas = 123.3 liters
Xmax = 32 mm
Xmax (parallel) = 32 mm
Xmax (series) = 32 mm
Sd = 125.6 sq.in
Qes = 1.1
Qes (parallel) = 0.55
Qes (series) = 0.55
Re = 1.9 ohms
Re (parallel) = 0.95 ohm
Re (series) = 3.8 ohms
Z = 2 ohms
Z (parallel) = 1 ohm
Z (series) = 4 ohms
BL = 1.922 lb/A
BL (parallel) = 1.922 lb/A
BL (series) = 3.844 lb/A
Pe = 800 watts
Pe (parallel) = 1600 watts
Pe (series) = 1600 watts
Qts = 0.89
Qts (parallel) = 0.49
Qts (series) = 0.49
1-W SPL = 85.8 dB
1-W SPL (parallel) = 88.7
1-W SPL (series) = 88.7

Gardner)