Frame, Stretch, Soak

No bass will be stuck in the trunk and cause rattles. All of the bass will be forced into the cabin of the car. Here's an extreme example of what I mean, I did this "blow through" for a pick up truck awhile ago, the box is in the bed (pictured without cover), and the bass will travel through the cut out into the cab of the truck.

The way this box is built it looks like since the subs are firing at each other that the sound waves would cancel each other out

Sound wave cancellation in this box design was irrelevent, it's a SPL box, most important was to have them all in the same phase. The equipment consisted of six JL Audio 12W7's and six JL Audio 1000/1 amps. With that amount of power and subwoofers, quality of sound is never gonna be a concern, it will never be musical. It's an SPL box and all that matters is how much pressure can be produced with this type of setup. By the way, this is a single cab truck!

There is no specific calculation for the opening, it's not a port. The opening is so I can direct the bass to wherever I want it (Inside car passenger cabin, not in trunk). I make the opening as big as i can, and as shallow as I can, so that any physical restrictions to the bass output is kept to a minimum.

There is more than one kind of bandpass box. The box in the truck above looks like a single reflex bandpass enclosure