Installed my Stebel on my SWM rack today. Cut a piece of aluminum stock to size, then attached it to the back side of the rack using the two bolts that retain the wedge-shaped Givi adapter. Bolted the horn to the inside of this, and ran the wires up the rack tubes then under the seat, where I mounted the relay to one of the tool kit retaining brackets. Fired it up, and it quickly shut up the group of teenagers making a racket out in front of my house. Paid for itself right then and there! This thing is seriously intense.
I live in Los Angeles, so I'm not too worried about water. It's rained maybe 3 inches all winter! I've had those bags on there for about 750 miles now, and they're still very clean on the backside, so it doesn't look like much is going to get slung into the horn.