need input on size of woofer opening in room

I need some advice on the required depth of an opening for a woofer built into a wall. My wife and I just purchased a cool Mid Century Modern home, with an angled wall facing the open vaulted living room. The back side of that wall is a guest bedroom that has the typical 1950's fitted shelves in the closets. This wall is 13' long..on one end it creates a void about 6" deep, on the other end it is about 30" deep.

On the living room side there is a full length audio "closet" where they had some kind of built in stereo originally. Because this part of the house sits over a crawl space, it seems like a great application for an infinite baffle subwoofer. However, what I want to do is to build a 3 way speaker into the walls. Because of the irregular size of the "Void" in the wall, I'd like to build a sealed box 15" woofer under the floor, and then vent it up and out into the room with a shallower box. For example, I'd like about a 7cf box under the floor, and wondered if the woofer could be mounted into a narrower box, say 23"w x23"h x 8" deep that would be sealed to the top of the box under the floor. Will this narrow box affect the sound negatively?

The work has to be done from the living room side, since I do not want to cut up any of the very nice, original wood that comprises the closet shelves and cabinets. I'd include a drawing of this, but don't have any idea how to link it to this site.

I'd want this woofer to extend up to about 900/1000hz to mate up with a JBL horn, by the way. I plan to put my tube amps into the original audio closet, and run a couple of dedicated circuits over there.

Thanks for any input!