Well, much to the dismay of my wife, I spend my entire day off building antennas instead of doing something useful. I built 3 antennas all exactly 40 inches across and 12 inches tall yielding an ideal 20 dbi of gain. The first was 25 inches deep, the second 10 inches and the third 5 inches. All were built from foamboard and Al tape (recycled as I went as I'm not made of money). I used the same Linksys wusb54g placed in each antenna's focal point.
The 25 inch one was a dismal failure, which was even more annoying as it took forever to tape down almost 6 square feet of that Al tape.
The 5 inch model worked very well, but the 10 inch antenna worked the best by a small margin. All were verified to be very close to a true parabola as I used a laser level to double-check the math. As the 10 inch deep antenna seemed to work the best, I finished it off. (almost)
http://img57.imageshack.us/img57/5962/test25eh.jpg
This weekend I plan on feeding the thing with a cantanna to see what that does, as suggested by Airstreamer. I can't yet say what the gain of this ... abomination is but I was able to connect to the target AP with no problem. (and can see almost 40 other AP's verses the 2 without the reflector)