We got the horn installed this weekend. My buddy designed a one-piece bracket that mounts on the left side with two screws and a small hardware part that I will have to identify on a future log-in. The beauty part is the actual connection to the horn, using a small hard donut of rubber similar to an engine mount sandwiched by the horn mount, bolt and washers. Very stout, but able to vibrate and give a bit without working loose.
Dang this thing is loud. My install has the horn outlet pointing straight out the front air grill for full effect. I will try to post some pictures.
PS I ran the black ground lead all the way back to the battery as you suggested, thanks.