Captured turnbuckle? Have you tried a net to catch one, lol. I assume you mean the ball ends. Have you replaced the balls? They might be worn or abrasive and wearing out the ends. I have never had one pop off, but the track i run on is not very dusty, so I do not get a ton of debris between the end and the ball. Are you looking to use a screw and washer on the top to prevent them from popping?