This is a problem that has plagued me for over 20 years. I was so pleased to see several 3D printed solutions. Unfortunately the original design by guss67 typically requires supports. To combat this there are designs that print in the wrong orientation. Or there is Old Lady's 2 part solution. I like it the best but it can slide backwards, which can be a problem when storing cables.

I've taken inspiration from another design and increased the bottom lip which presses against the bottom of the ethernet plug and prevents the print from sliding back. My remix creates a solid repair without needing glue or supports, and allows the flexible clip to be printed flat for optimal strength.

To assemble, slide on the base with the cut-out facing towards the rest of the cable. Then tilt the rear end up and slide the clip through till it locks into the base, slide the interlocked pieces back till it stops, and finally push the clip down locking everything in place.