In my experience a constrictor knot, whether with marling or wire and even if snugged up with leverage from opposing pliers, gets only limited tightness. If you don't use that lovely tool Brion advocates, use a quality stainless or monel siezing wire and a good steel tool (I use the eye in my Myerchin marling spike) to make the wraps really tight. They won't move then.
G'luck
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