You need to insert 4 washers while printing for the magnets to adhere to. For this stop the print at 3,4mm and insert them in the holes.

I used washers that were 15mm in diameter, 1,15mm thick and screws with a head diameter of 9,5mm. If these dimensions don't fit you, the .f3d file is parametrized for washer diameter, thickness and screw head size with some tolerance.