Leave the ends a bit long and buried under the knot itself. A couple of stitches with sail twine, or finer thread, where the two ends finish, tieing them into the other strands will secure it. Some put a thinned coat of varnish over the finished knot, which soaks in and aids in keeping it secure.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.