In my opinion, so long as it tightens against the head tube, it will develop sufficient friction to prevent turning. I built a home made luggage block that for my Swift, shown here, and it works well. I rubberized the surfaces that touch the bike to prevent damage to paint mainly but it also increases the friction. I have ridden the hell out of it and it does not turn. I think Valeria's works on the same principal so it should work. The lateral forces that would cause it to rotate are small unless the bike falls over, or something like that.