YAABB... Yet another Airsoft Ball Bearing :-)
Customizable for Your Needs.
Adjust the "tol" Variable according to your Printers needs.
A tol of 0.30 or 0.35 worked fine for my Printrbot Simple Metal.
Update: I also tried copper coated steel balls with 4.5mm diameter. Just adjust the "BB" variable to 4.5
Do you use FreeCAD? Look over to https://www.thingiverse.com/thing:2766243 for the FreeCAD counterpart!