These magnetic building toys use 8x2mm magnets.  Print the tips in two colours so you can keep north and south staight and you know which ends will attract each other.  The balls need to be magnetic. You can use steel bearings or print balls using a magnetic filled filament. I used Proto-pasta Composite Iron PLA.  I am currently using two magnets in the tip of each beam but if the balls are strongly magnetic then a single magnet suffices.  (ie the filled filament I used works but is not optimal.)