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Starflight Industries - Retrofit Motor Retainer
(High Power Rocketry - March 1998 - page 34 - by Sunny Thompson and Bob Sisk) 

   Starflight Industries has an inexpensive and simple retrofit motor retainer system. All you have to do is drill a hole in the aft centering ring, put a tiny amount of epoxy on the outside of the hole with a toothpick, screw in the adapter and let it dry. The adapter has metal blade that cut into the wood and secure it in place. Instructions are included but the system is so simple the only thing you really need to know is what size hole to drill. Very inexpensive, effective, and simple to install. The only think you supply is the proper "stove" or "fender" washer. You can pick these up at any good hardware store. You could spend a lot more to do the same thing, but why would you want to do that?