This is v2 to  my maker balls (linked here https://makerworld.com/en/models/80557#profileId-239957)

These use a 8mm by 3mm magnet for a stronger hold.

Also there is a hole in the top for a magnet. This allows them to be stacked.

I also have a pole with magnets on them. This allow you to place the markers on metal surfaces and not have a big 40mm holes missing from the scan. There hole will only be 10mm, Which most scanning software will auto fill.

The added benefit to the pole,  is it will allow you to get down low and scan upward. 

last photo shows how i install them. I made 2 stacks of magnets one N and other S. Pushed one piece of the print on one side and the other on the other side.

For the poles i simply push on N , flip and push on S.