What I did was glue a piece of plastic (the thickness of a piece of a cd jewel case) to the top of the plug. This created a snug fit and stopped the sagging. However, for added assurance I then used a strong rubber band and affixed it to the top of the handle I have on the rare of the amp. This has worked fine over the years. OBTW, thess are all things I had laying around the house.
Good luck with your solution,
Lou