get a bunch of small round magnets with a center hole and a length of steel banding material roll the banding into a cylinder use a rubber band to secure it place the magnets around the perimeter until you you have the desired number adjust the banding until the magnets touch all the way around if the banding makes 2 or 3 complete laps it will remain reasonably cylindrical in shape with all of this placed on a steel plate you just need a transfer punch to locate the centers of each magnet