I want a louder horn
That little "beep, beep" of the OEM horn, IF heard by anyone, only makes them look around for a clown car. Since you're not in a clown car, and you want people to acknowledge your presence, a better horn is in order.
You can get a LOUD FIAMM horn at Harbor Freight for $10, or $15 at a regular car-parts store. Places on eBay have them, as well as JC Whitney. They are also available from Nissan dealers as part #25620-ZB000, with a list price of $9.70. That's cheaper than you can usually get one from a parts store.
Get the low-tone, and make sure you're getting the loudest one they sell. It should be listed as 120 or 130-ish dB. They're way louder, and are a bolt-in replacement for the stock horn. Point the bell of the trumpet down so it doesn't collect water, and cover your ears before you test it in an enclosed space. Even with it pointing down, it is loud and can be heard by cars and other motorcycles easily.
The low-tone horn is still relatively high-toned. It cuts through noise quite well. The difference is that when people hear a deep horn, they get more of that instinctive "Oh crap, what's going to smoosh me?" feeling. You want them to have the feeling that they're impinging on your space. It seems to produce a more pronounced effect than the high-tone horn. When you hear that low-tone horn, you look around for the big Cadillac that's about to turn your car into junkyard art.
Some members tried the high-tone horn, on the theory that people would be looking for a bike if they heard a high horn, but it didn't seem to work out in practice. The low-tone horn brings much better results. It sounds more like a semi than the high-tone.
It's a direct replacement. No relay is necessary. It includes a bracket for mounting. Installation time is estimated at "120 seconds".
The top picture shows the stock horn on a naked bike. The bottom picture shows the new horn installed on a bike with a fairing.