I put a set of the Harb Frt $10 horns on my 2001 Toyota Highlander. (For an SUV, it had a horn comparable to an early 70s Honda 100. WIMPY!!)
The kit included a relay, but no wiring. Still, was able to mount right behind the plastic grill, about an hour's work. On a motorcycle, my concern would be with the mounting studs possibly being too fragile. But for $10...?
P.S. -- I had one of these mini air horns on my Kaw Nomad, built in mini-compressor, no hoses to fool with:
https://m.harborfreight.com/bad-boy-air-horn-94117.html.
A bit ugly, but LOUD...for more $$, it's available in chrome.