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.
Use a rubber band in between a stripped screw and your screw driver to give it enough grip to remove. With all of the Rainbow Loom bands scattered on the floor in every room of my house (I have a 9 year old), this shouldn’t ever be a problem.