Extruded the base down a bit in meshmixer, then capped it off and inserted a void for an 8x3 Niobium magnet using Blender. There should be room for 3 layers above and below the magnet. Pause your print at 3.8mm, or if you're using 0.2mm layers it's layer 19, insert magnet, and continue.

This is a remix from https://www.thingiverse.com/thing:398515