This magnetic holder uses a ball which is meant to work with this locking ball and socket system:
Four magnets are used, two on each side. They are each either 20mm x 10mm x 4mm or
20mm x 10mm x 5mm neodymium N50 magnets bought from eBay. Search for "20 x 10 x 4mm neodymium magnets". Note that in one order, I received 4mm ones and in another order, I received 5mm ones, I don't know why. For that reason, you'll see that there are two sets of 3D printer files available, ones for the 4mm magnets and ones for the 5mm magnets.
Nylon nuts and bolts purchased from Home Depot hold them in place (search for "Nylon bolt" or click here #6-32 3/8" Phillips-slotted oval head machine screw and matching nut).
No brim is needed.
Note that the OBJ and STL files were exported from Blender. Cura was able to open the OBJ file but not the STL. I assume this is a Blender issue.