The Old Britts piece is very nice.
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.