More updates. I got the $3 ebay horn (the stock one was huge, ugly, and would have been more difficult to reloate) mounted up and under the tank. It won't be visible when the tank is on, as seen in the profile shot. The aluminum plate was made extra long, so I can cut a horizontal slot into it to accept the tongue I'll be attaching to the nose of my seatpan.
I also mounted the voltage regulator outside of my custom electrical box, as recommended by many DTT members, for better cooling. It also frees up some very limited space within that box, which will house my mini battery, starter relay, fuse block, etc, as well as other necessary bits like my ignition switch, starter botton and horn button.