I drilled a couple of small holes side by side about half way in from the end of the thumbstrap, and tied a small loop of leather lace through the holes. Kind of hard to explain, but it makes a little tie on the underside of your thumb so it holds the loop tigher around your thumb. You can adjust the tightness of the thumbstrap by tightening or loosing the loop tie. Make any sense? Works great. Holds my thumb completely from slipping in the loop. I can put a picture up somewhere if anyone wants to see it
Just put a rubber band around the thumb loop to make the loop more snug. Position the rubber band so that the thumb loop fits snugly, and wrap the rubber band around the thumb loop enough times that the rubber band stays in place. Works great.