I ride a quiet motorcycle with loud electric horns - been doing so since 1978.
Cagers and others are not expecting a freight train so it's fun to give em a blast.
Be careful as some sociopaths get very angry when getting a dose of Decibels
The use of a relay is required not an option - They will be louder as they will get full voltage/current this way.
You do not want all that current arcing burning your horn button contacts
LOUD Horns can be the difference between crashing and not crashing so take your loud pipes and put them in the dumpster as they don't work.
Mounting them is always a challenge but I fabricated a bracket the works well.
I will post up a pic later today.
Mount the relay where the horn lives-if you are really vane you can put it in the cowbell [remove the orig horn....doh]. I have a harness I fabricated that integrates the horns to the motorcycle nicely &
I use the battery tender feed for the horn power - the horn circuit runs the relay which is mounted where the cowbell mounts.
The heat is run off the accy circuit wiring behind the side cover using the harness that you have to get from HD.
Hella horns used to be my choice of horn but since the manufacturing went off shore the quality went down - over time the the horns need replacing from getting soaked.

 

ADVNTR, please post some pics when you get a chance. I'm still waiting for the Fiamms horns to arrive but have been scoping out the bike to see exactly where I want to mount everything. I have my relay and inline fuse wiring, just need the horns.

 

Decide what you expect from any horn. To be the most efficient, the horns need to be facing the direction you want and with no or little obstructions impeding the sound. I chose the Fiamm high/low tone horns. I mounted mine on the front frame with chrome brackets. I think I paid about $40 for the hardware and relay. I used #10 stranded for the wiring as stranded carries more current, which is what you want with horn(s). Don't forget dielectric grease on the connections. In addition, I put heat shrink over the connections too and have had zero issues with connectivity.

Now I have something to offer back when I get cut off.

 

I mounted mine the opposite of those posted above. In other words, the one on the cowbell side, I put on the other side of the engine. This keeps the horn outlet down to reduce water from getting in the horn, along with a little tilt down also.