Really like the model, very easily printable :)

I simply added a 9mm diameter 3.5mm high internal cavity 0.6 mm above the bottom to allow to fit a 8mm diameter 3mm high neodymium magnet inside.

Simply add a pause gcode at the start of the last layer before bridging over the cavity so you can manually place the magnet inside. A second magnet under the printbed can help to keep the embedded magnet in place until bridging is complete. I use an M600 filament change gcode since my printers support this code.