I assembled a manual boost controller based on a design by Craig Watson. The pipe fittings and other assorted stuff were picked up from the local Home Depot, the spring and tubing were easily found at Pep Boys, and I hit a local bike shop for the ball bearings (I used two, I didn't trust the spring not to slip into the bolt thread). Total cost was about $12, which is a far cry from the best price I could find (about $50 for a decent ball-and-spring MBC). I've only done some basic dialing-in of it so far; I'm running at about 15 PSI fairly consistantly now, but there's a very unnerving spike of 2-3 PSI at WOT (which quickly subsides). I'll probably do a little more tuning later when I've got a chance to do it during the daylight. ;-)