I have just fitted one of these, I mounted it on a bracket just behind the oil tank.
It came with a fantastic wiring loom and relay and when fitted looks just like a factory fit job. The bracket took ages to get right though.
I sourced mine from Australian ebay as I don't think they are available in the UK due to a 132db noise level. There is virtually no delay, not like the old air horns of yore, press button 2,3,4, parp!