There are some potential pitfalls… you should probably use the ACC wire from your ignition switch. The ALT wire usually goes low during starting and glowing (if you have a diesel), but is otherwise high when ACC is high. Also make sure that you wire the power from a different source if you want a clean shutdown.
Reading the OnOff shim description, it triggers on a low signal, so switching off ACC should do the trick. Not sure if it works the other way around, for switching it on with the ACC.