There is a post in there explaining that washers didn’t work because they just spun (not sure why…).
If you use round washers then yes it spins as you clamp the cable up, use square ones or a small square block of Alu and it buts up against he locating ridge and works a treat, costs about 10p from your local hardware store.
You can machine/file a step and the pokey bit if you want a better fit but a simple square block or couple of square washers works fine too and isn’t any bother past the first 30 seconds of tightening the cable up.
If you want to be extra frugal and save the 10p, you can use one of the concave spacers from an old v-brake pad, file one side flat to stop it spinning, and then file a tiny tiny groove in the top for the cable, also works a treat and costs nothing but 2mins with a file.