Take up space so that you don't have to fill box with more dirt than you really need? Figure out how many inches of dirt you will need. Mark the inside of window box to how high you need to fill with other stuff to take up the extra room. To fill the space you can use any kind of empty plastic bottles, empty sandwich containers, cans, what have you to fill that space. You can use Styrofoam or wadded up newspaper to keep the weight of the box down as you need to place it outside the window. Anything you do use, you should make sure it is clean. No need to attract bugs or whatever.
I'll use a filler on the bottom, such as some old small plastic pots or even the plastic tubs the flowers come in (you can cut it to fit). Then, fill with dirt and flowers.
All of the answers above are great. I also used gravel and sand. Adds weight tough so if that is a concern for you don't do it. You could also buy some of the cheap styrofoam sheets from somewhere and cut them to size to place in the bottom of the boxes. I like the stone and sand as it allows drainage for the plants but that is my preference.
I use non-biodegradable packing peanuts that I save from any package that has them. I will say that fewer companies are using them now though. However, there are a lot of other packing materials that will work. I just save them during the year to use in my window box. If you use something that's non-biodegradable it will stay for years and you'll only have to do it once. Wishing you the best.