Cardboard boxes make terrific brooders. I'm glad you found a nice big one. I would open up the entire box to the chicks. They may be tiny, but they do love freedom of movement.
I also caution you that by age two weeks, a couple wise guys will discover how to use their tiny wings to achieve lift. If you don't want to go on a chick hunt, it would be best to cover the top with something. I found that cheese cloth makes a great topper as it will stretch nicely and still allow you to see into the box.