You may want to check what the load bearing capacilty of the factory QR Sissy bar with locks is. Manufacturers usually underrate but that is potentially a lot of mass, leverage and wind loading on a non bolted system.
It may be covered here.
https://www.triumphinstructions.com/ProdDocs/A9758067-EN.pdf
I have one here and had a quick look at the mechanism before I wrapped and taped it all up with half a roll for shipping some time and do not feel inclined to unpack it for pics.
@Mittzy may know.
QUICK RELEASE SISSY BAR - CHROME A9758067