End of the New England sailing season. IN anticipations of next year's fun, am making up a tent for the boat's cockpit (over the boom, down past cockpit coamings on each side) and will stress the covering material with a loop of 1/4" dia. shockcord that will travel from edge of covering to a nylon tube with snap that locks onto rubrail's underside. Want to be able to adjust the shockcord until I've got it right, then tie it off for good. Tried a square knot which quickly traveled as I tightened it. Is there one knot that does the trick or many?