First I'd like to thank Spunkmyer for his idea on where to mount the Stebel Nautilus horn as he described in THIS post. 

From a safety standpoint, I was most concerned with the horn hitting the fender under heavy braking and thereby restricting steering inputs. The other concern was reduced cooling air to the radiator, but I ride on the cold and wet side of Washington state where we don't see high temps in the summer. I'll just have to keep an eye on coolant temps when I venture to warmer climates. And because I ride a lot in the rain, I wanted to make sure the horn could drain easily.

By holding the horn in front of the radiator, I could see that it would be possible to make a bracket that would position the horn close to the radiator and tilted back so that the fender could not hit it and rain could easily drain out of the opening. I next cut a piece of cardboard to get a rough idea of what shape my bracket would need to be (shown on left in picture below). It's real easy to trim with scissors until it looks like it'll work. I then used my cardboard as a template to cut a scrap piece of sheet metal so that I could actually/sorta mount the horn and get the bends correct. Now that I had proof that this idea would work, I traced my pattern on a piece of 1/8" steel. A little cutting, drilling, filing, bending and painting yielded the final product (on the right in the picture below).

I should note that I had to do some fine tuning on the bracket bends before painting it. The positioning is critical so that the horn won't hit anywhere on the radiator while turning the bars lock to lock.

Here's a front view of the horn installed. I haven't hooked up any wiring yet.

And finally, here's a side view of the horn installed: