I finally received the 25x10 mm magnets from China. They are stronger than I thought.

The distance between the magnets is 20 mm. From the video below you can see that they hold quite strong.

In my design the body has a total shell thickness of 12 mm, maybe I can add a couple of mm to get extra stiffness and strength for the sphere.

BTW... the plastic parts are a prototype for the R2D2 rocket boosters, but that's another story... :)