Sometimes things just pop out, slip out, fall out, or give out when you are racing.
You would not be the first one to need to replace the concave chock shaped for a 2-1/8 in. (53mm) diameter mast. This chock is 3/8" (10 mm) thick and complements Ronstan's flat mast chocks.
Some dinghy sailors like to have two to ensure that they have a snug fit around the front and the back of the mast.