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).
You may want to take a look at the other version.
Google Maps Marker - Magnet 1 (without hole)
Google Maps Marker - Magnet 2 (with hole)