Used to have one until I changed out my controls which used a different thread. I'd hang my helmet by the clasp then slip a little padlock over the stud. The piece of crap helmet I have probably doesn't interest anyone now anyway.

I have an old Shuberth helmet that used a fork type fitment. I'm thinking of stitching a loop with a D-ring on it, to the strap to use the common helmet lock setups....