On Touring bikes look at your front tank mount. They all ready are the prefect size for a low cost loud car horn to be mounted.
On my RG I mounted one on each side. The car horns come with a small flat bracket to mount them. They are out of sight and easy to wire up.
The hole in the bracket is the right size for tank mount so there is no drill needed.
Small enough,out of site, easy to install, loud and low cost win all the way around

 

I just got a Stebel Nautilis off ebay, loud as he!! I got the Nova chrome model for 64 bux, looks pretty good in the regular horn position. It will not fit under the cowbell. You can buy a Stebel in black for 38 or so and seperate the comp and horn as mentioned earlier. At 139 decibels, the Nautilis is pretty decent.