I was thinking it might be cheap enough to hide under the fender in the unlikely event you lost your keys away from home.
Just like the old days, the key in the little box with a magnet on it. You used the hidden spare key when you shut your keys in the car, which was kkeasily done in those days. I had to use it one time and had forgot where the spare key was hidden, I finally found it and the box was just about rusted away, in a short time only the magnet and the bottom of the box would have still been there.