If you think the standard SD horn is pants, I've just fitted the Stebel Magnum TM80/2. The /2 refers to a twin horn conversion. These are electromagnetic horns designed to sound like air horns. The advantage is they're smaller than air horns, although still quite bulky, and they blow immediately you touch the button.

You can fit a single horn without the relay. It's still an improvement. Two are a big improvement to standard without bursting your eardrums, but loud enough to get you noticed - a nice fat twin 'air horn' sound.

Discrete fitting is straight forward - one screws directly to the bottom threaded bolt hole for the standard horn bracket (gets warm down there but horn polypropylene body hasn't distorted). You'll need to drop the battery box slightly so you can spin the horn on without it hitting the exhaust. The second, including the relay, bolts neatly into the gap above the Oil Tank filler cap sufficient to still get access to remove the filler cap etc. You'll need to drill a hole in one of the Stebel brackets provided so you can bolt it to the top fitting of the coolant bottle. Take particular care to mount it away from the rad fins!! With the tank & belly panels back on, you can't see them.

Wiring takes time to hide away properly as you can't get a fag paper in anywhere - otherwise it's a straight forward mod.

The Stebel wiring diagram is wrong. Here's one I did earlier:

Nice one Pete, sounds like a simple alternative to the Stebel Nautilus customisation others have done.

Yes - mounting both horns & relay really is straight forward....just takes time to neatly route the wires from where the original horn sat to above the Oil Tank. Otherwise, piece of cake.

Eastern Beaver kit useful re wiring - viewtopic.php?f=1&t=12862&p=148306&hilit=+stebel#p148306

i'm using the Stebel Nautilus airhorn, the way i wired it - the way i wired all airhorns on all my bikes/cars for years, is:
positive from battery, with 15-20A fuse on the wire going to relay (pin 30), comming out from relay (pin 87) and going to the airhorn;
negative from battery/ground directly to the airhorn
two wires from original horn to the switching leds on relay (pin 85 & 86)

it's seems like the same thing, but... the original horn's negative of course is ground, but it is a thin wire, ment for maybe 5-10A, i would worry about it overheating when running 15-20A through it...