I found some similar tools, which were too “complicated” for my requirements so I made this new one. Just insert two 6x3 or 5x3 magnets while pausing the print at the appropriate layers. I reccomend to glue them in, so the printhead doesn't pull them out while printing over it.

Now you have a standard for the polarity of you magnets :)

If you need other sizes, let me know!