Another simple 'lock' are velcro straps looped around wheels and frame. The op's thief would probably have jumped on and fallen over, never noticing the velcro. He would have noticed the helmet. In a pinch, I've used string in my DF days. I am rarely in a thief prone area, and on a loaded bent, not a big concern. 'Course the serious thief would throw the bike in a truck and be gone.