This is for a heavy 10L, but maybe it will give you some ideas. I used 1/8" dia rare earth magnets (four), mounted in 3/8" dia aluminum pucks epoxied to the gear. They are sized so that the magnets would pass very close to the Hall-effect sensor. The sensor was mounted on an aluminum tee bracket affixed to a spot I flattened off and added a tapped hole. This sensor was part of the accessories offered with the old MachTach kit which unfortunately is no longer made.
-Gary
