This cap is designed to attach 16mm corrugated flexible cable conduit to NEMA 23 stepper motors. My stepper motors are about 56.2mm across faces, and the conduit used is 12mm ID and 16mm OD. There's a 4mm slot to make it easy to insert cables without removing connectors. The SketchUp files are also provided in case you need to modify.
I made these for a Proxxon MF70 milling machine with CNC conversion, but obviously they are generic and could be used elsewhere.