I use a small Kryptonite U lock on mine, it's really all of the lock that I need because I don't commute to any especially rough areas right now. I simply loop it through the front loop of my rear rack and lock it (the front part of the rack that runs perpindicular to the line of travel), and it more or less sits over the rack/panners. Occasionally it gets jarred enough to slide forward and into the line of travel of my feet, but this is rare, since the roads I ride on are fairly smooth. If I were really worried about this, I would get a small bungee cord and loop it around the back end of the U lock to the rear part of the rack.