Hello. As a fabricator of various audio components and furniture as a hobby, I find your question one that I 've given some thought to if I owned a home. I've designed a few very large, but unobtrusive enclosures that would utilize the basement, walls, and ceiling joist areas.
Concrete is my favorite material being relatively inert (I'm in the process of constructing a pair of dipoles in concrete, as soon as I recover from my accident). There are only two things to consider: The rooms usage and furniture and the listening area (sweet spot? a couch..chairs?). From there I start with the mouth of the horn and work backward.
The walls will act as the horn mouth, both corners of 8' height and into the room down the wall approximately 5'. I would fill the areas between new studs of commercial steel with crete. Then double 5/8" fireboard sheetrock to the areas making up the horn mouth or final flare. The corners of the room should be decorated with "least deflection" furniture; tall spiny/spindly plant stands, art, and mike boom light stands etc. The port to the main body of the horn is through the floor in the corner to the basement where you can do what you please to facilitate the expansion rates, flare etc. The speaker enclosure itself can be of any suitable material you choose. Fold it to accommodate your basement environment.
This came as a modification of the Klipschorn. HF drivers I put in the ceiling. Here it depends on your home. My idea is to use the area between the floor joists as the HF horn. Time delay can be all the way to next door if you have nice neighbors! Two joists a side would have to be removed and repositioned to form the flare rate. The mouth would be a small cabinet a few inches from the ceiling and could be decorated at it's sides to camouflage it, or some fine hardwood stained etc.
Just some thoughts. I have two "W" enclosures with 16" Altec Lancing Woofers in an enclosure 6' x 2.5' x 3' deep. I use these for my recording studio and pro-sound reinforcement. At home as a supplement to my theater system, they reside against the couch wall 9' apart and are finished in a Cherry veneer. Grilles of steel rod bent into rectangles and covered in the largest most durable pantyhose my girl could find. They look nice, but are heavy! I do have to hump them around on a two wheel cart. Think about it!
