Have exactly the same problem. I was so worried when I was out in a F4/5 the other week, that I roped off the auxilliary, fearing similar damage. Still to work out which will be better - using a pulley system, or a short ratchet strap to maintain sufficient tension.
Ideally I'd have the auxilliary onboard fixed to a custom mount, but as space is at a premium, it has to be mounted on a transom bracket. The auxilliary tilt-bolt is poor, but given it can be used as the main engine on a small SIB, it's probably sufficient for purpose.
I'd be interested in any pictures from members regards how they resolve this particular problem.