Old Britts carry a bracket (13-000001B ) that you can rivet to the top edge of your MkIII side cover and then use the 1972-74 Dzus fastener (062489) in the existing frame hole. Alternatively, you could either replace your side panel with one from a 1972-1974 and use the Dzus fastener or make your own bracket from sheet metal and a Dzus spring.
Years ago I used a piece of plumbers tape, which is a coiled strap material with holes. I wrapped it around the main frame tube isolated with foam, used a stove bolt with a wing nut, extended it over to the side cover and attached it with another stove bolt and wing nut through the rubber grommet. I was always going to clean this up but never did. You could not see it unless the seat was up or off. I sold the bike with it. As far I know it is still there.
Rocky Point also has a 'd' clamp that similates this. I am sure it looks better too.