Reuse Your old 2.5'' HDD Magnets on Your magnetic pinboard or fridge. 

Make sure Your Magnet is not thicker than 2 mm, this is the maximum thickness for this model. You can use two Magnets stacked (for more magnetic force), if they are not thicker than 2 mm in total.
I tried many different Magnets from 2.5'' HDDS to make sure they all work with my design. If You find one which doesn't fit, let me know. 
You can also use an 20 x 6 x 2 mm rectangle Magnet. 

Printing:
Print the model in the orientation as it is with 0.2 mm Layerhight.
Add a pause at the beginning of Layer 13 (2.6 mm) in Your Slicer.
Put the Magnet in the designated hole, while the Printer pauses. You can add a drop of glue before putting the Magnet in, if You don't want it to wobble around in the finished Object. 
Resume the print and let it finish.