For a .40 size plane you can use the standard nylon horns, place back-to-back, and drill through all holes same size as clearance holes. Now you will have four holes through both horns. Use 2-56 screws that will go through the horns and thickness of rudder, and secure with some Nylock nuts or regular nuts with lock-tite. I like the Dubro horns, as they are a little sturdier than other brands of same size.