Thinking of a horn or large ported boxes. Still not sure. Might have to build both and see.
The thing to watch with a bass horn is making it too small for the cutoff. If you shrink a horn too much the price you pay is ringing.
Andrew, I have lots of ideas like that last one. One of my favourites is the entire front wall removed so it is a giant straight bass horn. The side walls are merely an expansion, so the "loading" goes beyond corner loading, but you sit in the horn mouth. Not only would it extend very low, but it would launch a plane wave (like a double bass array which is on the blog), meaning all room modes are eliminated except a few length modes (which would be really bad ones needing the entire back wall to act as a very good bass trap). In the ideal case, the back wall would be an exact replica, digitally time aligned and inverted in polarity to actively cancel all remaining modes. So then you would have a very low cutoff sub bass horn with unlimited output working as an anechoic subwoofer with no room decay issues at all. Does that sound like mumbo jumbo? In laymans terms, lots of good technical reasons to build something huge that is way over the top and sounds really good, and that will make you an audio forum nerd celebrity with everyone on every forum posting pictures of it about once a year.
Question. Are you going for the circus tent look? Otherwise I'd be thinking about getting into the corners. Think of it this way. Imagine a bass horn suspended in the air by a chopper. Really big! Now let's sit it on the ground. We call that half space. It now radiates into half of the infinite space, if that makes sense. We gain in theory 6 db, and the horn mouth can in theory be half the size now. Now let's add a wall behind it - again we've cut in half the "space" into which it radiates, horn mouth cuts down again, 6 db gained (again in theory, not quite in practice). We call that 1 pi space. Go one further step and add a side wall, so that the horn has 3 boundaries around it. Now we have corner loading, called 1/5 pi space, it's the same deal.
Now let's consider this ...
Question .... which loading is given to that bass horn in the middle? That would be 1pi loading! Looks cool in the brochures, and in this case makes it easier to fit into the room. What is that thing? It's actually quite simple. It is basically a modular bass horn, with 6 pieces of quarter pie. Each has IIRC a pair of 12" subwoofers, and they have designed them to work to about 40 Hz. Below that point, the above has 12 x 12" subs with lots of power, so it can fill out the bottom octave through sheer brute force of lots of drivers and power. It's a clever design, with curves you could construct one with some sheet metal that is easy to bend into shape, perhaps laminating a few layers joined with construction adhesive, until it's strong and well damped enough. And if getting CNC done you could cut a groove in the end plates. It need not be a difficult build. I was close to building one of these, until I did a quick 3D render of it in my room. Ugh! Too much space taken up! Dang! BTW, you could do a nice version with some Rythmik servo kits.
Now for those with a big room, or a big-ish room ... split it in half and put those in corners if you can. Using just 3 of them instead of 6, in a corner, achieves the same result, due to room loading. Use 6 modules in the corners (3 in each) and you effectively get the equivalent of double placed in the middle. Or, coming back down to earth a little more, just put one module on the ground in each corner, and put your horn on top. It's probably going to elevate your sound stage a bit, so you want to have a mid horn on top of that and probably not something like the trio.
Seeing that pic (Andrew), I had your other spot in mind, with the plantation shutters, now I see why the size restriction.
Back to your midbass horn ...
First idea that comes to mind is making the sides parallel with some ply. You lay it down on it's side when building, tracing out the curve then putting blocks so that you can then guide and bend say some 3mm MDF in laminations into the curve. I'm thinking the curve is probably gentle enough. Now the problem might be at the throat, where the aspect ratio gets a bit stretched. Hmmmm, there goes that idea. My next idea is to contruct it in 3D cad, so that you know the way to cut the four sides, with them all curved. You know the area at the throat, midpoint and mouth, and you go for a continuous curve that ties them together, whilst including the 90 degree corner bend. If you can model that, then you can cut the four sides out of 3mm MDF. Then join them together at both ends, throat and mouth. Now you lay it with the mouth facing down. Now cut a block that is the exact size of the mid point, propped up on a pole so that it is positioned in the middle of the horn. Could be tricky. The idea is to clamp all the walls to that piece, running glue along the edges, then pressing them together until they sit around this internal spacer. You would probably need to rig up something, maybe with occy straps to hold it together whilst the glue sets, in various places.