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.
If the location is near the user, you'll need to tape over the ends so nobody gets cut on the hardware.
On second thought, this might actually work better for the job you describe - plain old stainless steel worm drive hose clamp. Still needs taping over to prevent cuts if in an accessible location.
