I have one. Details:
http://contour.org/forums/Forum5/HTML/000775.html It can be done, with the right driver. I won't recommend it with most 8" woofer because it won't be worth the time.
Dave Newman on here has heard it and was impressed. It just adds enough extension that the doors cannot handle. Sound quality dispite free air is excellent. I don't listen to music with low bass hits, but it has happend a time or tow and even I was impressed how clear it sounded.
