This is where it all started for me, to have something I could use with a Clari-T I had to buy. Here's the original recipe:
http://www.tnt-audio.com/clinica/bfb_e.htmlAs a starting point, Google tells me 45 liters = 1.6 cubic feet, or about 2,765 cubic inches which translates to a 14" cube. Of course, don't build a cube but that gives an idea.
I think they are most un-boxy sounding and tremendously dynamic. They aren't the last word in sparkle or subterranean bass but they are surprisingly adept at both ends and are not outclassed in my Altmann/Lamm/RWA setup.
I built mine with a slanted front baffle which was a pain to build but works great to utilize floor reinforcement in the bass and still "aim" the driver at the listener.
They are ugly critters but solid as a tank. I stuffed the box around the driver to further minimize back wave energy. I have speaker wires soldered directly to the driver, so it's amp-direct - no interceding anything.

Let's say I have the utmost respect for real cabinet guys.
This photo shows with the Fostex 206E which was recently replaced by a used, first-gen Omega driver. I picked mine up used. I asked Louis a while back if I could buy some from him, but he said the drivers were OEM only. So, I'm not sure what the best driver option would be. That would be a beneficial direction for this thread to take.
Anybody interested in the single driver idea wanting to try it out, I wholly recommend this as a starting point. In fact, that's exactly what I did with these very boxes. These speakers changed everything for me. Since building these, I abandoned my previous direction. What these do well, namely micro- and macrodynamics, big time instrumental tone, and coherency are the stuff music is made of.
A sheet of mdf or plywood, a little sweat equity and a pair of drivers and that's it - they punch WAY above their price class.