I use a mini evo 0.9 kg. This will not secure your wheels and frame and is difficult to lock to big lamp posts but if you use security skewers for the front wheel and a sheldon style locking method you get good securely for a lowish weight. You can not fit a hydraulic jack in the space left. The worst point is that you are restricted as so where you can lock. I use this method in London and never have too many problems finding somewhere.