Take a look at the last photo:
link. You can see the bracket, and one of the disc horns. I also used the aluminum bar to mount the oil cooler. This arrangement put the car horns facing forward, and slightly down, but out in the wind, so nothing blocked their sound.
Try running a wire directly to the battery, for power to the horns; then try the horn circuit. This will indicate whether you need a relay, supplying power directly from the battery, to get maximum sound out of the horns. The OEM horn circuit may drop too much voltage, making the horns sound poor. Relays are only around $5 at any auto parts store, and they're simple to wire in. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII