Helmet lock

I use a small combination padlock like the MasterLock 630D. Slip it through the D-Ring and over the brake or clutch lever. It's long enough to go over the lever, but too narrow to go past the ball end. My fullface HJC will then fit over the mirror so it ain't upside down collecting rain or errant Starbuck's cups.
Tiny enough to fit in my jacket pocket. No keys to lose. Always with me no matter what bike I'm riding. Cheap.
 
Tried that one, Paul. It wouldn't fit anywhere except on the handlebar. That turned out to be too precious a site to waste. I've got lots there and tried many other locations. It wouldn't even fit on the sissy bar, so I finally gave it up!