Me too. I hang the helmet over the mirror, open-end down to keep out the rain. I use a simple cable-type luggage lock that I got from REI. It cost about $13.00 or less. One end of it is locked around the mirror and the other end is for the D-ring. Most of the time, I never lock it, but just insert the end of the cable into its receptacle. (A thief is not going to spend a lot of time messing about with the helmet. He would grab it and run; except with the D-ring locked, it won't come off easily. This is the system I use if I"m in a public place for just an hour or so. Overnight, I'd probably never leave the helmet hung over the mirror, even if it was locked.What I was thinking of, was to put the helmets on the mirrors and lock them somehow. That would ensure open side down and they should be stable.