I live in a terrace house which directly fronts the pavement of the
street. Three years ago I put three stone window-boxes on the window
sill at the front of the house to brighten up the house and the
street. This year, for the first time, someone has stolen one.

What can I do to secure them? Are there any off-the-anti-theft shelf
systems I can buy from a gardening centre or DIY shop?

Many thanks in advance for any suggestions.