Thread the metal rod through the PVC pipe. Adjust the rod so the 90 degree bend at the end comes through the narrow slot. Cut a half inch piece of wooden dowel. The dowel should be wide enough around to fit snugly in the hole drilled in the PVC thumb control. Drill a small hole in the center of the dowel. Fit the dowel snugly over the metal rod and glue in place with Zap or two part epoxy.