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)
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).
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)