I wanted something nice that I could use a set of existing neodymium magnets I had. So all I did was enlarge the magnet openings. That did cause a slight weirdness on the short side by the dice holder when I exported it from Meshmixer. But it printed fine.
You might want to up the infill, I ended up popping one of the magnet recesses into the model when I was installing the magnets. They're just a press fit for 6mm diameter, 3mm depth.