@swan4356
From the MUSBJIM files of "Keeping It Simple":
I'm not sure how your stock horn is installed in your VM Speedster, but here is how the stock horn is mounted in my Kirk Duncan VS Speedster... (Click on pics to enlarge)
...and here is how my auxiliary air horn is mounted in the passenger side of my VS...
As you can see, I mounted my air horn using a couple of corner brackets...
...and mending braces...
...to fabricate (i.e bend or twist) a "custom" bracket for my air horn. These really inexpensive items can be found at your local hardware store.
Hope you find these pics useful as a visual reference on how you can easily fabricate a "custom" bracket for your specific need.