Please provide a link to the magnet case you use. I have had a much different experience than you. The first time I used a magnet case hidden on the frame, it fell off. At the time, I was doing a lot of freeway driving at about about 75 mph. Perhaps freeway driving is more likely to cause the magnet to fall (?).

With a little thought, I'm sure I could make a more rugged device than a magnet. Anyway, I have no problem carrying a spare key on me at all times. It's more peace of mind this way.