Not that anyone asked......but here's how I mounted the, "Screaming Banshee" horn. Easy to buy, not so easy to find a place & bracket to mount to. I Used a piece of 1" flatbar, drilled holes in each end, bent it in a vice and well that's it.
If you're looking for a louder horn, I would suggest you look at the Banshee. Pretty interesting story behind the company, crowd_funded. I get .15 seconds of regular horn, and then the air horn kicks in.
I've only used it once and the person that pulled out in front of me held it tight to the left and was in on-coming traffic lane. I think he thought he was about to be hit by a truck.