Very simple to print, should work without sanding, but not promises there, mine worked fine. It is designed for 16mm marbles or ball Barings if you so wish. I will be possibly changing its look, making it a little less raw and removing some of the sharp edges, but that's only if i get time.

btw I haven't checked if there is already something like this on Thingyverse already, if there is... please make mine :)

If you have any issues, feel free to lmk of said issues.