I'd love to take a crack at a DIY project. For example, how about the 3" Tangbands in a sphere? What about that 4" Fostex Ed Schilling uses? Tannoy Dual concentrics!?
Funny you should mention the Tannoys, as Tannoys-in-a-sphere is on my list of possible projects. Way back, I made a '(very) poor mans Gallo' using one of the Rat Shack Linaeum dipole tweeters perched atop a Seas mid-woof in a homebrew sphere (plastic light globe covered with plaster- UGLY). Sealed, so no bass at all, but yowza did they image like mad.
I have a pair of the Tannoy system 600's which use the 6.5" dual-concentric. Yeah, not the coveted high-efficiency units, but still about as good as it gets for co-axial drivers as far as I can determine. The two possible projects they're slated for are
a) "MTM" with some 7" drivers and a homebrew transient-perfect digital xover (much easier)
b) in a sphere but still with a TP xover. (way harder, but serious cool factor).
I'd been thinking of roughing up the sphere with hexagonal donuts cut/spliced from standard 2x4's, and then either routing down the excess with a jig of some sort, or else just building up/filling in with Bondo or something. Building/Filling would probably be easier, and would probably be easier to finish.