Hey George,
Just get that on board air setup, a small pressure tank, a solenoid, and a diesel truck air horn! We had a similar setup on my Dad's old F150, at first with just an air tank. When I installed it I went over to my uncle's house to fill the tank, and he was working on his car at the time. I started filling. When I reached 100 psi, I hit the horn. He looked up, nodded, and said it was pretty loud. At 125 psi I hit it again. He banged his head on the hood, and his cat jumped off the roof of the car and took off. I thought that was probably loud enough. It certainly made traffic move!