This is an attachment we have used for a model crane that we adapted to be controlled by a Raspberry Pi. The attachment has a cylindrical hole inside the structure, into which a magnet can be dropped. In order to drop the magnet into the print, it is necessary to pause the print at a print height of 4.5mm (this can be done in Cura by using the 'Pause at height' plugin).

The magnet we have used is 18mm in diameter, and 3mm thick (http://cpc.farnell.com/duratool/d01768/magnets-rare-earth-18-x-3mm-pk3/dp/TL14071), but a smaller magnet could be used if necessary.