I grabbed your Mio 4 spot holder rack and added a simple backing wall to accept 4 to 16 of my 3x10mm (2.7 x 9.9)mm magnets. I use a 0.6mm nozzle so I made the clearances a bit more than I usually would. And I printed 4 little rectangular plugs to add into the top after inserting the magnets. Using only 4 magnets might not be strong enough so i jammed in 2 magnets into each slot. I'll secure the magnets and plugs with CA glue.
I will probably reprint it in PLA with a smaller nozzle since this 0.6mm nozzle and PETG is quite stringy.
EDIT: Added a version with 8 slots total. I wasn't confident in the holding power so this should allow for up to 16 magnets if needed. 16 may be overkill but it definitely holds them :)
The author remixed this model.
Thanks to the original designer! I added a simple backing wall (in Tinkercad) to accept 4 to 16 of my 3x10mm (2.7 x 9.97)mm magnets so I can mount it to the refrigerator door.