Attaching Window Box?

In our front windows we have enough room to put flower boxes. On the parlor floor I am ok just resting them on the sill (not too far to fall). My problem is while I would like to attach them on the upstairs windows, I don’t know how to keep them secure.

We have flyscreens that prevent us tying onto the window guards. I definitely don’t want to drill into the stone sill. Drilling into the wooden brickmolding doesn’t seem ideal either. What have you done to solve this problem?

Many thanks.

By going4broke |