This is a very good grinder but the protective lid has lost, hence the replacement. It prevents the beans popping out during milling.
Or if you need to to push some colour into your kitchen :)
Technical
It's a very simple design and easy to print, with a well tuned printer it's going to be dimensionally accurate as it is. you need a very nice first layer to get a nice finish.
Material: doesn't matter, PLA holds perfectly as would ABS. Maybe a semi-flexible filament would give the best result, I haven't tested that.