I arranged it very similarly to what you have Haggis on your twin, mine being on a new bike in 1973, but I arranged the loop (on mine it was oval) to be off centre and very heavy on one side, I then tightened the loop so that the heavy side was at the bottom, it took a bit of careful filing to get that bit OK, but this was my solution to it not coming off, on the basis that if it shakes loose it has got to get past 180 degrees before any more can occur, I never saw it move at all, on the inside I made a plate (well more of a thick sheet) with a nut welded on it that was also fixed by the top suspension mount, new bike! what me weld a nut on??? OK, you guessed it, I welded a nut on! but it was three years old at that point.
CLEM