I'd forget the Rivco horn and truck down to your local Murrays auto parts or Autozone or Pep Boys or whatever and get a Wolo
www.wolo-mfg.com air horn or a Stebel Natulis. Both are cheaper and I believe the Rivco is a Stebel, marked up.
The Wolo and (I believe) the Stebel will fit under the sidecover on the
R3 standard and classic. Wolo calls it a Bad Boy, Stebel calls it a Natulis.
And no, I don't sell either on my website. I believe you can buy off the Wolo site.