diffraction-free speakers

If rounded surfaces are what floats your boat, you could also consider cylinders:
1) PVC pipe--available in a variety of diameters and thicknesses. If you can't find the 1/2" wall stuff, use two cylinders, one inside the other, and fill the space with sand.
2) Cardboard tube: Available under a variety of trade names, but also shows up in unexpected places such as the center tubes in rolls of carpet or linoleum, etc. or centers of large cable spools. If you're in the right place at the right time, you can get large cardboard tubes for free. (Not a benefit to be overlooked...)
3) Lucite is available in large tubes in some places. It isn't going to be cheap, but it would offer color options that might appeal to you.
Note that edge diffraction can be handled quite well by simply radiusing the edge of a conventional cabinet with a 1/2" or greater router bit and/or by covering the surface around the drivers with sound absorbent foam or felt (thus preventing the sound waves from ever reaching the edges).
A fringe benefit to using tubes is that, once capped, they will stand nicely on their own.

If I recall correctly, Chris Venhaus uses the Gallo Nucleus Sennheiser References as the reference speaker for his system. His website can be found here: http://www.geocities.com/venhaus1/index.html
You can probably e-mail him about more information on them.

Also, another man started a spherical speaker project out of mdf, but never completed it. He was making the enclosure by routing out a different size circle for ever layer, and then putting all of the layers together, so that the outside of the cabinet was box-shaped, but the inside was spherical. He never completed the project however, and their is only a brief description of it here:

His Utopia clones are also impressive, and I currently use Chris Venhaus' design for silver interconnects in my system, and the Cat-5 cable design for my HT.