A big problem with the Sonoff POW is the cover that comes with the device. It does not close fully when the cable jacket is inserted into the strain relief "teeth", and when the jacket is left out of the teeth, no strain relief is provided.
This design addresses this problem by providing strain relief with 2 customizable jaws which can be adjusted for cables with different diameters.
You will need two M3 16mm long pan head screws and nuts for this.
Make sure the support direction is perpendicular to the shell lines, or you will end up with a a very bad finish on the inside