There are two ways to refer to：

Someone ask to buy the RTT rail lock, it is useless, as the weak part is your cross bars not the rail itself.

First: Strongly recommend Apple Airtag, no monthly fee.

How does Airtag work? Your AirTag sends out a secure Bluetooth signal that can be detected by nearby devices in the Find My network. These devices send the location of your AirTag to iCloud — then you can go to the Find My app and see it on a map. The whole process is anonymous and encrypted to protect your privacy.

2nd: Use strong steel crossbars,

2 steel bars, U bars mount on the side rail of the car, though labor expensive, if you are very handy, do this way.