Just got a Screaming Banshee to replace the anemic mouse fart of the OEM horn.
Was able to fit it under the left side cover. I replaced the M6 bolt that holds the fuse box and battery tray with a longer one and double nutted it on the back with the supplied bracket.
Ha! It's not ideal placement, but I didn't want to see it, and that's the only place I could come up with to hide it. That's the upside of how bad the OEM horn sucks: even sub-optimal placement is superior to the factory meep-meep.