I have tried some of the magnet placement tools while printing my first Gridfinity parts.  They helped getting the polarity correct, but I frequently glued the magnet to the tool by mistake.  This tool has an internal retractable magnet.  You squeeze the syringe to grab a magnet, then place it and release the syringe.  The tip of the tool remains holding the magnet down until the internal retention magnet is away.

I use it to place 2mm x 6mm magnets.  I have not tested other sizes/strengths.

Assembly is straightforward. Glue the 5mm retention magnet to the end of the plunger.  Slide the spring onto the plunger.  Insert plunger into shell.  There are press-fit openings on the plunger and shell that gently retain the spring.

If you use a larger or stronger retention magnet, I cannot be sure there is enough travel to ‘drop’ each magnet when the plunger is released.

There are two versions of the shell, with and without fingers.  I've included the f3d file.

Necessary Parts:

• 1 x 5mm round magnet
• 0.6 x 7 x 20mm spring

Printing:

• Print the plunger with thumb side down, no supports needed.
• Print the shell open side up
  • Use supports about 10mm from the bottom to prevent it falling over
  • If printing the shell with fingers, additional supports are needed for the fingers.
• PLA is fine. YMMV