I use a small separate magnet, intended for magnetic door sensors. It's the same size as the one that comes with the case, and is a small, white pill shaped, self adhesive item. About $3 each.  See here https://www.aartech.ca/vip120-2c/flair-pill-style-surface-mount-door-window-magnet-only.html

This is what it looks like https://www.aartech.ca/vip120-2-flair-pill-style-magnetic-contact.html

Works great, gap can be 15mm and it still works. I have photos in my post on this forum about my door sensor (see here http://mcthings.createaforum.com/mc-innovations/front-door-sensor-via-mqtt/. I now have my door sensor in an MC case, but use the same magnet.