This parametric motor retainer uses the "Poor man's openscad screw library" found here: https://www.thingiverse.com/thing:8796

The settings in the scad file are adapted to my printer, aside from the dimensions at the top, you may need to adjust the "fudge_factor" setting until you get an appropriate fit on your body tube and a well-functioning thread.

Use this at your own risk, if you do not have sufficient layer adhesion, this part will not retain your motor when the ejection charge goes off. Ensure that your printed part is sufficiently strong before using it in flight! If the threads are too loose, they can slip off the retainer ring, so be sure your thread clearance is tight enough as well. The clearance should be as tight as possible while the threads are still functional.

Note that not all adhesives work well with printed plastics! Be sure you have a strong bond between the retainer ring and the motor mount tube before you use it for flight.