Using a 1/4" drill bit, drill a hole below the rivet where the buckle had come off. Be careful to drill only through the flange and not through the hull. Install the buckle by inserting the nylon screw in the hole and attaching the buckle behind the flange using the nut provided. Tighten with a screwdriver.
Pro tip: add tape to the back side of the wrench to hold the nut in place.