Magnet size: OD 15 mm, thickness: < 2.4 mm
Just glue a small magnetic disk in the roof with BT's plastic glue (Super good on PLA)
Link to the detector:
https://www.biltema.no/hjem/sikkerhet-i-hjemmet/brannvarslere/brannvarsler-2000043498
The author marked this model as their own original creation.