You have me thinking about a setup (box) like the one JAMAS has -- with an internal sub. (Could fire forward, backward, or down...IDK)

One big box...a bit higher though.

Lid opens with speaker inside....but protected with a large wooden "L" to box it into a corner. But this "L" wouldn't span all the way from front-to-rear. That way, the woofer would be [both] open to the box, but protected from internally stored objects. Maybe some screen to complete the internal chamber.

I'm not sure if internal objects (as I describe above) would have much determent on sound level. Maybe it could/should be more open...say with 1x2's and screen? Maybe a horn-loaded woofer in the box...with only an outlet?....Hmmmm...Space for the sub too?

Seems it should sit up as high as the seats -- when folded down.