Securing window boxes

[SimonFJ]

I'm hoping to put out some window boxes on the top two (three floors) floors. However, the property is in quite an exposed area jsut by the sea and can be quite windy (plus the occasional storm blowing up the channel). There is a reasonable amount of pedestrian traffic passing by so am concerned about the window boxes potentially falling and causing injury.

Besides putting heavy stones in the bottom, does anyone know of a method of securing them down or indeed whether bars could be placed across the window to act as a barrier to secure the boxes down?

Any suggestions or ideas welcome!