Hi Tom,
I got one of those steel horn mounting strips that you get with some cheaper horns (about 70mm long and 20mm wide with a hole drilled at each end). I folded that in half (so then only one hole at one end, drilled another hole at the other end in the now thicker and shorter strip of metal, and then bent it in a dog leg (bend to the left, right and left again).
That then goes on a bolt from the petcock mounting. I used rubber washers so the bolt had to be longer than the one it replaced.

Its shown a bit wonky in the picture, but its easy to adjust and now sits at a perfect 90 degrees.
I put a rubber bush on the bottom end that sits on the engine casing, taking some weight and reducing vibration.
So far so good...

Dave.
Tomorrow its three gauges from motosens to fit (radiator temp, volts and clock) plus a blinker buddy...
Got a Gear change indicator coming from Holland next week!!

Ride safe.
HH