Here's how I did the ones on my 1/5 Cub/L-4. There are several brands (DuBro, Sullivan, Ernst, and others) of pull-pull setups available in different sizes, or you can use coated stainless leader line from your local bait and tackle shop. The ball joint connectors and threaded rod is available separately also. The rod has a small hole for the cable to go through.

Note I crossed my wires; it allows max throw and arm movement while maintaining equal tension. You can also use different size arms between the servo and rudder for increased or decreased mechanical advantage and still maintain equal tension.