I use a narrow hasp combination lock from Lowes, Ace, whatever. I think it's a Master Lock 630D if memory serves. You want the hasp to be wide enough to fit over the blade of the clutch or brake lever, but too narrow to fit over the ball at lever's end. Cheap, small, no keys to lose, one lock for all bikes. On most of my bikes it works to lock it to the lever then set the helmet on the mirror so it doesn't become a trash or rain receptacle, otherwise I can put it over the 'bar end.