Kleinn 6127, made for the roof of a pickup and in no way looks out of place on side of the bike, I have even had a lot of people think it is stock (but I wouldn't assume they knew much about bikes).
It's nice for knocking people's cell phones out of their hands because they think they're about to tangle with a large truck when they hear it as they drive along completely oblivious to everything around them, good fun! It's also good for clearing pedestrians and even deer off the roads.
I'm sorry, I left off the most important part. It is a Stebel Nautilus Chrome Compact Motorcycle Air Horn. I also bought the universal wiring kit, which make hook up unbelievably easy. I think they rate it around 130dB. I don't know if it gets there or not, but it is loud!! They say to mount it within 15 degrees of vertical, but quite a few people mount it horizontally, and don't have any issues. So, that is what I did.
check with your local constabulatory department as these are highly illegal in many parts of the civilized world canada for one. same story with blue and red flashing light