Overvoltage Protection Circuit

So it seems the guy that I got the idea from.... Doesn't know what he's talking about...lol.

I think I'm gonna just leave circuit the way it was before I got the bright idea of complicating it way over my head.

But In the future I'd like to be able to "Protect" a circuit that would be installed on a vehicle.

This seems like something that should be straight forward, and done all over the internet, but for some reason I can't seem to find it.

What would you guys do to accomplish this?