It's a rather sad fact that many people who are brooding baby chicks in the confinement of a plastic tote indoors have not realized they could be brooding right in their coop or run, and their chicks could be enjoying all the freedom associated with having a spacious place in which to grow and play and explore. As long as chicks have a heat source, their brooding space can be any size, even 100 square feet.
If this is your first batch of chicks and you have no adult chickens in the picture, and you have a coop already built and ready for occupancy, all you need to do is make sure it's secure from predators, weather, and cold drafts, install your heat source, and move your babies in and watch them thank you by running, flying, jumping up and exploring even the highest perches.
Baby chicks confined to a box are missing out on the best time of their lives. And it's all because we think brooders are the way you are supposed to raise chicks.
If your chicks are trying to escape the brooder, they are trying to let you know that they need space in which to be chickens. Do them a favor and let them move into the coop now.
Even if you have adult chickens in the coop, you can still partition off a safe pen, either in the coop or out in the run, as long as the run is protected from the weather, and the flock will begin to accept the chicks as full members. This makes integration a little later a breeze.
Think about it. Why are your baby chicks in a small brooder box indoors when they could be enjoying their coop and all that terrific space right now?