I really liked this idea, and had a need for it. But I increased the cup diameter to 70mm and modified the magnet end for some magnets I had lying around.
marker cup mag bar.stl
This is for 51mm x 13mm x 5mm bar magnets
marker cup mag.stl
This is for 19.05mm diameter x 3.1mm thick disc magnets
Probably would have been better to make it one piece like the other, but I didn't use it so I didn't fix it.