Looking for a better sensor that I can use on my mailbox....

I purchased the Smartthings smart sensor yesterday, and upon attempting to attach it to the inside of my mailbox and mailbox door, the way the 3M tape is set up on the sensor, it would not allow there to be a decent contact between the sensor and its magnet. I seem to need the magnet’s 3M tape relocated. I tried adding extra 3M tape to a different location on the magnet (which I attached to the inside of a mailbox door), but it still wasn’t working out. My niece saw someone in a white suv open our mailbox for no reason the other night, so that is why I’m wanting to add a sensor to it. TIA