The easiest way to install the retainer is to push it into place attached to that longest motor. Then insert the 4-40 set screws, and unscrew the motor. The next time you put the motor in it should screw right into place.
NOTE: We do not recommend permanently "glueing" this retainer in place. Epoxy does not stick well to PETG, and there is no great way of positioning the retainer without the motor in place until the epoxy hardens.