The search term for those cabinets would be "ripoles".
Sort of like a folded-up OB. Gives you a bit of extra path length (and slot loading), but they start to get weird above a few hundred Hz - bear in mind you're effectively listening to the 90-degree off-axis response of an 18" cone.
Drivers look decent. I've got some SM112s around here somewhere that won't be used for much longer. Might be a good match if you're interested.
They are called Ripole - similar to the Linkwitz dipole, but with higher compression. Axel Ridtahler pioneered this variant. There are several documented builds from Germany. https://www.lautsprechershop.de/index_h ... ler_en.htm