For securing a grille over a window, i think i could use a strong openable ring instead of a padlock. If it needs two hands to open, it couldn't be opened from outside by fingers through the grille. If it's easy to close, you would be more likely to do all your windows, as it would be easier than padlocks (also should be cheaper). My idea for a design would have the ring opening to form 2 joined half-rings, with a snap to close, needing some force so 2 hands would be needed. I'm not sure of the details, though. Does anyone know if such a thing exists?