I modified the original STL file from Norwegian1 http://www.thingiverse.com/thing:444649 and added two holes for neodym magnets. I used N52 neodym magnets with a diameter of 10mm and a height of 5mm (http://www.banggood.com/10pcs-10mm-x-5mm-N52-Strong-Rare-Earth-NdFeB-Neodymium-Disc-Magnets-p-988441.html).
MagnetWhiteboardMarkerHolder.stl | 428.8KB |