I have an extra bar going from transom to transom of each hull, but instead of that you can also use a fixed line instead. In the middle of that bar there is a ring which is conected to each end of the bar with a bungee cord. So the ring can move freely over the bar but wll always return in the middle.
My safely-line is a standard leash line from a kite-equipment (infact it is a so-called handlepass leash, because they are longer: 2 meter). So the leash is connected to the ring and the other end to my harness, both with caribiner hooks.
The trick of that leash is that there is a safety release on it. So I can always detach myself, also under load!!!!!!!

The line goes over the tillerbar but under the joystick.