This is my first time on this board. I live in NYC in a rental building that does NOT permit alterations to the window frame or sill to properly secure a window box. Does anyone have any recommendations about what type window box set-up would possibly work in this situation? Are there any brackets available out there that might hook into or in some way make use of the window sash to secure the window box? Any ideas would be much appreciated.

YardBirds (western US) has some nice looking hooks/holders for pots of different sizes. They aren't real cheap, but they look nice and hang pots over the outside railing.

While I'm saving money for more of these , I've bought some L shaped *rafter hooks* or hooks for holding tools in the garage or workshop. You might find a way to use them.

The url included here show a picture of a rafter hook.