Aosu WRM Core Magnetization Kit

To be specific, 3x2mm magnets refers to 3mm in diameter and 2mm tall.

For magnetizing one cube:

Print 8x wrm4x4corner.stl

Print 4x wrm4x4core.stl

For both files, I recommend -0.12 XY size compensation, and 0.15 layer height. No supports are necessary. I printed on a Prusa MK3S+ so you may have to slightly modify these parameters with a different printer.

Next, take two of the 3x2mm magnets and glue them together. Then glue it to the corner insert and glue the insert to the corner piece. 

Then, glue 3x2mm magnets to the inside of the ball core segments. Each ball core segment should contain two magnets. Make sure the polarity is correct so that the corners are attracted to the outside of the ball core

Finally, assemble all ball core segments (they fit together by grooves) inside the cube, and reassemble the cube.

Reassembling a 4x4 is annoying, so you can disassemble one half, glue in the ball core and corner inserts, then reassemble that half and repeat on the other side. This saves you from having to do a full reassembly.