Is your coworker limiting his search to electric?
I use an Airzound on my recumbent.
The horn mounts next to my left thumb and is very easy to access (it's just like all my motorcycles). The tube from the horn to air reservoir is about 3 feet and was pretty easy to mount.
The small bag on the front, just behind the BB, holds the air reservoir (I don't remember if the bag came with the horn, but I think it did).
https://oi26.photobucket.com/albums/...pse0a5c81b.jpg