My friend Jeremy lost his Garmin cadence sensor magnet so he replaced it with a simple neodymium magnet. After hearing about how well that worked for him I picked up some similarly sized ones from eBay and put one on the crankset for my new bike. I opted for a 8mm thick (10mm diameter) magnet where he’d gone with the 6mm thick version. I picked this based on how far out from the crank arm the normal Garmin sensor sits, although with the additional strength of the larger magnet this may not have been necessary.
The magnet sticks solidly to the end of the pedal spindle, nestled inside of the 10mm hole in the Race Face Crank Boot. It’s a bit difficult to pry off with my finger, but easy enough to remove I can’t help but wonder if I should have opted for a thinner version instead, so I just purchased some 3mm thick magnets on eBay and will give those a go as well. Whichever I end up using it’s a very elegant solution that I expect to work out quite well, as long as it doesn’t come out and become stuck in my drivetrain or something terrible. It also looks much nicer (and is much easier to fit) than the normal Garmin magnet, as it doesn’t use adhesive nor require a cable tie around the crank arm.