This house was one of three chosen by a couple on HGTV's "Fixer Upper", based out of Waco, TX. Chip and Joanna Gaines redid the house from top to bottom. I love what they do, and I'll hopefully talk them into redoing a house for me someday :-)
Pea gravel and pebbles can weigh down windowboxes. Empty water bottles are a lightweight option. Place rinsed bottles inside the planter and add soil on top of them. The space between the bottles will allow proper drainage without the extra weight.