If it's 1/8" use a 1/8" bit, drill a hole as deep as the magnet, (not counting the coned part), mix some JB weld, dab a little on the magnet making sure to get the sides. Now put it in the hole and tap it down until it's flush with the surface. Take a center punch and put four punches about 1/32" from the magnet, it won't come out. Make sure you have the polarity correct if you put it in the wrong way the sensor won't see it.