I just retrofitted my Estes-era NCR Archer with one of these retainers. The motor mount extended just far enough to fit the retainer perfectly. It was also quite snug, which boosts my confidence in its effectiveness. Unfortunately, the cap extends below the airframe and thus makes the rocket unstable (obviously not aerodynamically) when sitting on its tail.
I have flown these retainers three times - twice on my Ventris and once on my pre-Estes NCR Big Brute. Neither showed any evidence of wear, and they shouldn't as long as the rocket isn't placed too close to the blast deflector. My only worry is long term heat degradation of the 5-minute epoxy that I've been using. Maybe I should have waited and bought some JB Weld. I guess that I'll just check the retainers prior to each flight. On my scratch builds, I've been placing the motor mount flush with the bottom centering ring. I now wish I hadn't done so. I like these things!