This is a magnetic base for the modular mounting system.  The design uses two rectangular 1" x 1/2" x 1/8" thick magnets.  I used p/n BX082 from K&J Magnetics, but any good neodymiums with those dimensions should work just fine.

Option 1 - Embedded (way cooler, but requires a little gcode magic)

There are two designs included.  The one labeled "embedded" requires the print to be paused so the magnets can be dropped in before being walled in with filament.  I followed this example using Simplify3D and it worked like a charm!  It also includes instructions for Cura and OctoPrint.  Since each slicer is different, you'll have to find the exact layer number yourself by slicing it and stepping through to find the last layer before the holes are covered.

Option 2 (less cool, but printable as-is)

The second file labeled "slots" includes slots that remain open so you can print the thing completely and then slip in the magnets.  I haven't actually tested this one, but wanted to include it for people who don't want to mess around with pausing the print.  I'd suspect that if you use decent magnets, their attraction to each other should keep them from falling out.  If not, maybe a little glue might be your friend.

Print Orientation

One last thing.  For stength, it's best to print this on its side using supports.  The magets are pretty strong so they might rip right through like this if you print with the base down.  Then again, I 'm totally guessing, so go for it if you're feeling wild!

How much can it hold?

With it stuck to the fridge, I was able to hold a phone on an arm that was extended out about a foot, but YMMV.  It doesn't do super great when subjected to a torque that rotates the base around the mounting point.  At least for me, there wasn't enough friction to keep it from rotating.  Adding some electrical tape to the bottom helped somewhat.  Some kind of adheasive backed rubber might be even better.  But keep in mind that the magnetic field strenght drops off pretty fast with distance, so you'll be giving up pull strength the thicker you make it.