Fiam dual horns are available through most auto parts supply stores and are not expensive. They are very loud and bolt right up. I used the mounting hardware from the original horn, and hung the second horn from the speaker hole. I mounted the horns and pulled power and ground from the original wiring. I didn't use a relay on the oilhead and the horns have worked great for 10 years. I have mounted these horns on a half a dozen other bikes, most included relays.
Still a learning newbie !
I went with a Stebel
and a separate harness. I made a harness / relay on my Yamaha years ago when I added a 2nd horn, fused of course. It has worked out well.
I'm going to do 42k maintenance next month, am going to wire it in while the tupperware is off.
Thanks for the advice all.