I saw one city park where they took this one step farther. They had a small wooden structure (I think the cover to a water valve, from years ago when they used to set up an ice skating rink). The open it so rarely they don't even padlock it - they just bend open an S shaped link of chain, put that where the lock would go, and bend it shut. When someone needs to access it, they cut the one link of chain with a pair of bolt cutters, and then put on a new one.