Oysterman's stopper knot for synthetic shackle

[Tinkerer007]

I tried your knot, and it does look like a nice stopper, but it does not seem too persistent. It can be undone easily.

I read somewhere on the internet that a modified Turks Head knot is used. I am tying one now to get familiar with it, and then will investigate which modifications should be done. It doesn't seem to be easy to tighten this knot though.

A question I have is about the tail. It looks like the synthetic shackle is done with two tails. But most of these stopper knots are done with only one piece of line. Are you burying the one tail inside the other?