I have had the Mini Beast by Pro-Pad and it was LOUD on the RKC as well as Howards Horn.
Kuryakyn puts out a air horn set-up that mounts to the inside on the saddle bags.
The Kury Hidden Horn is not a air horn, but rather an electric ones that sound like an old Buick. Although they aren't as loud as Howards air horns, they are a bunch louder than the stock HD POS.
I have the Revco Air horns mounted on an L shaped bracket bolted to the jiffy stand bracket. I had pictures posted of that setup in the old forum. As we all know everything was lost............
I have installed the Rivco air horn on my 07 RKC and the wife's 06 sportster 1200C. LOVE IT!! This horn saved my bacon last friday!! Very load, and the car moved right back into their lane. HIGHLY, HIGHLY recommend Rivco. Great customer service too!
The installation took me about an hour on my RKC, and about 45 minutes on the wife's sportster. But I"m very, very picky on how I did every step. But overall pretty easy to do. Good instructions come supplied too!