I went to fly my Estes Sidewinder, Saturday, and when I put the engine in (standard size mount, standard size engine), it slid right up inside the mount tube several inches. It was bound to happen, the kit had a serious design flaw in it, right out of the box, IMHO. It came with the regular standard size Estes Engine Hook, but not only was there no mylar sleeve to hold the hook to the tube (you were told to use masking tape to hold it down), there was no thrust ring included with the clip. The hook was the only thing stopping the engine thrust.
There was no salvaging the motor mount or body tube. I was able to salvage the fins and decals, and I'm making a better lower section for my Sidewinder. Improvements include bigger 24mm E size mount instead of the undersized 18mm, piston ejection, and I'm canting the fins a little more than original, to try and get more spin. I'll also use a larger shock cord and just 1 chute instead of having the nose come down on its own. I stuck with an engine hook on the motor mount, instead of the plastic retainer because the retainer really killed the look. I added a proper thrust ring and mylar sleeve.
I've been adding thrust rings to my builds that don't have one for the motor mount by taking expended Estes motors, cleaning out as much of the residual BP out of them with a flat blade screw driver as possible, rough sanding the outside, and the using a hacksaw to cut them into .25-.5" lengths. Can get 5-6 of them out of any given motor casing to throw in a bag/box for future use if you don't have an official thrust ring on hand.