For a magnet with a dimension of:
Ø = 5mm
h = 2mm
I used
N52 magnets which have an approx. adhesive force of
650 gram or
1.433 lbs.
e.g.
100pcs 5mmx2mm N52 Magnet (external link).
Recommended Glue: UHU Max Repair Extreme (or something similar)
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UPDATE: I slightly improved the model structure (v1.1) and added some extra space for the glue to make the assembling easier.
The shown images are from the previous version (v1.0).
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I recommend buying "RED Strong & Flexible" and paint the indentation in a dark color or just buy the other version with the real hole.
Google Maps Marker - Magnet 1 (without hole)
Google Maps Marker - Magnet 2 (with hole)