Is there some way to attach a bracket to the wooden part of the window? Depending on how deep the sill is, you might be tempted to just let it sit -- but nothing spoils your gardening vibe like killing a passing pedestrian!

A word of advice -- prefab window boxes *never* seem deep or wide enough to keep soil moist. I always either made my own (for a one-season planter, you can make it cheaply, or even for free!) or used a ton of the soil moisture crystals.

What are you planning on growing? Even in the most urban environments, I always had some herbs growing -- even if it was just some mints and chives. Last summer I was walking thru Over-the-Rhine in Cincinnati, and noticed one of the buildings had window boxes with petunias and vinca vine in all the windows of the upper floors. The vinca looked like it was clothig the building -- beautiful!

You could use double sided velcro. It has sticky on both sides. Attach one side to the sill and the other to the window box? As long as you installed it on a dry day, it should hold.

Is there room to grow inside at the window, on a table or something?