TIP: get a cheap key dupe made at the hardware store and drill a hole through it (or get your fave local handyman to do it). Then place said key behind your license-plate with screw through hole. This will keep it secure and it’s rare that you’ll be stranded without something that can be used (dime, piece o’metal by road, helpful person’s screwdriver) to remove the screw and access the key. This $2 fix has saved my bacon many times. Those magnetic boxes are OK but they can dislodge when you hit a pothole.
Brilliant.However, as other commenters noted, if you live in an area known for license plate thefts or your key has a theft-deterrent device in it, you’ll want to explore other options.