Helmet Locks

I just use a regular run of the mill padlock (combination type so no key to lose) and lock my lid to either the brake or clutch lever. The ball on the end prevents it from slipping off. I can get 2 helmets on the lock I have. Nothing fancy but it works
 
Never locked up a helmet. Just place it on the bike and walk away. Had a guy one time try to walk away with my helmet, Ask him if that was a good idea.
 
Just saying. There are some folks to $uck with and there are some that a helmet just is not worth the trouble.