I saw a few magnet applicators here but, for my needs, seemed overly complicated for what they were.

PRINT IN PLACE

STL and STEP files included.

I knocked this up in 5 minutes in FreeCAD. Each side accepts a 10mm x 3mm magnet. Make sure you orient them the correct way - it doesn't really matter which way, as long as they're opposite. It's 60mm long and will fit in 2x gridfinity boxes - or attach it to your printer! The possibilities are endless.

I've labelled each side "1" and "2" because I thought that would be easier to remember. Positive, Negative? North South? Seems easy to get mixed up (at least for my brain anyway!)

So the idea is to pick up magnets with the "1" side and apply them to one side of the box or whatever you're doing. Then pick up magnets with the "2" side for the lid or whatever. Simple!

It's print in place (the step & stl files should be oriented correctly, with the 1 and 2 facing up).