The B&C came with an M5 threaded hole in the back and I just routered a relief for the D220. It's a snug fit for the D220, however I wanted some extra security.
The only problem that I can tell right now is that my imaging is a little low - when I'm feeling lazy and laying on the couch the image is perfectly centered (to my ears). When sitting up however the image is low and I seem to be able to tell the speakers apart. Thankfully the whole baffle/stand cost me only about $10 ($6 for the 4x4 and a few for the threaded rods). The rest I had laying around the garage from previous builds.
I'm thinking about tilting the whole thing a little and seeing how that comes out. Else I'll have to see how much higher I need to get the drivers - crude eye-balling is ~5 inches or so.
Additionally - the bass from the 21s seems rather well integrated without localization (or at least very very little). I don't know if it's because of the low frequency or because of the nature of OB. Maybe someone else can shed some light on the matter. Without the 21s, the mid/tweeter is seriously bass-less - much contributed by 1) the small baffle and 2) the nature of the woofer itself.