Somewhere I saw a house that had a long, 12" to 15" wide wire-topped table under its first floor windows. It was set with lots of red geraniums in same-size pots. Very pretty. Easy to water because of the wire, and it would be easy to replace an individual plant if needed.

So, not a true "window box" but it avoids the "how to attach it" issue.

Following. Having same dilemma