I have a part that I'm going to get printed in plastic at shapeways. It is a brush holder, as in a carbon motor contact brush. So I want the brush to be able to slide in a square hole. If the brushes are 3.92mm, what size hole should I put in the part? I was thinking that I would design parts with different size holes, but it would be nice to know where to start. Probably get it printed in versatile plastic to start, and then professional plastic when I'm happy with the design.