I use ‘Suck it and See’ mirrors on my ST. You would think that they wouldn’t be very secure but I have towed for thousands of miles with them on and they have never come loose. They also have a thin elastic cord which slips over the mirror in case the worst should happen.
If you position them carefully then you can still have some vision from the existing mirror, you can also use the electric mirror controls to adjust them from your seat.
Conal