With the centerboard up, get a piece of line long enough to go under the boat and up to the deck on both sides and back again. Run the line under the boat from the bow, with both ends on one side of the boat and the middle of the line on the other. tie off one of the ends and loosely tie off the midpoint on the other side of the boat, with the line running at about the middle of the CB. Now with the remaining loose end, tie a giant loopy knot around the other line. It needs to be a big knot and tight within itself so that it won’t come undone, but loose enough to slide on the other line that it’s tied around. I don’t know the name of the version of this knot I’ve used, but doesn’t really matter. You could even thread a small float onto the line if you want to get fancy with it. Now, go to the other side of the boat again and tighten the end of the line that’s tied off to the boat, so that that end of the line is tight and pulling up hard on the centerboard. Finally pull the other end of the line (from the loop side of the boat) so that the knot slides down the taut line and under the boat. When you feel the knot come up against the CB, pull just a little more to put some sideways tension against the CB, but not enough to pull the knot over the CB. Tie off and you are done. The CB shouldn’t wobble back and for the anymore with thebknownpuahing it to one side...
When ready to sail again, just untie and remove the line, drop the CB, and you are good to go.