3D printer stl file for 71mm to 55mm adapter. Suits many low cost spindles which can be sourced from China, a good alternative for European builders.
This one is a 400W 48V unit as used by the Ooznest chaps in the UK.:
CNC 400W Brushless Spindle Motor ER11 & Mach3 PWM speed controller & Mount + PSU
(Note the matching end stops, a resource from "thehans")