Overkill, but that's OK, - made for "wire rope" don't go overboard on tightening or you might damage plastic rope, but these are simple and inexpensive. Should be fine for just keeping a knot from coming loose.

[![u-bolt rope clip image from uscargocontrol.com][1]][1]

If the location is near the user, you'll need to tape over the ends so nobody gets cut on the hardware.

  [1]: https://i.sstatic.net/YTXqL.png