Looking at the Stebel site I noticed the non-stock horn sold by Old Britts (
http://www.oldbritts.com/13_032010.html) is identical to the Stebel TM80 horn aside from the labeling on the black plastic part of the horn. They're both made in Italy so it's probably just a name change somewhere along the production line.
Here's some shots of how I mounted mine:
The horn comes with two little sheetmetal "arms" with holes on either side which you can kinda see in the first two pics. One end of each mounts to the big stud in the center of the metal side of the horn and the other ends can go wherever you can find two other bolts to line them up with. It just so happens that when you use the oil filter mounting bolts on those two free ends, the horn ends up tucked nicely between the frame tube and oil filter. The only thing you need to watch is if there are large divots in the rear engine cradle where the center stand stops hit it when it's up, the center stand might come up far enough to contact the horn. I haven't had a problem with it though.
You'll also most likely have to fashion a wire or two to replace the stock horn cable harness since the original won't be long enough. It comes with a small cable to connect the ground spade to the center stud to ground it so theoretically you'd only have to run one wire, but I chose to run a second wire to use a ground connection on the harness itself.
If you really wanted to you could use the center stud to mount it in the stock location, but it's nice being able to take it off or check the connections without disassembling the whole bike...