We used to take 1" nylon webing, about 8" long, and make two loops in it. One was big enough to go around the fin strap, and the other was big enough to put a finger or two into the loop. I took them to a shoe repair shop, and had them sew it with nylon thread in the middle. It functioned in the same way, but was not subject to deterioration that an O-ring would. It didn't stretch either. I still have some over 20 years old.