Why should the MPR and HPR guys have all the fun?? One of the things I LOVE about my MPR and HPR rockets right now is the screw-on motor retention. There's something very satisfying about screwing that motor in there nice and tight. At the moment, I'm taking a short break from building HPR stuff to get my park flyer squadron back up to speed which includes rebuilding an Estes Mini Honest John that met an unfortunate end when it's chute didn't deploy. One of the things I HATED about that kit was the motor retention strategy was just "tape it up 'till it's tight". To me, that doesn't feel nearly good enough so I started looking for a 13mm screw on motor retainer. Turns out that doesn't exist so I made one. 
 

Make SURE you print these on the finest possible setting or the threads wont come out.