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If you are worried about the rubber slipping on the railing and you take that more seriously than possible scratches in the railings finish you can use contact cement on a cloth backed sand paper glue it to the rubber where the grit is toward the rail. When you compress the rubber with the fitting, It will have a bunch of little pressure points that will bite hard. (This will mess up your finish if you a. dont tighten it up enough allowing the grill to move, b. put it on and remove it often or c. Someone falls against it in choppy weather causing it to move)