If you have an old window screen or something like that, I would cover the box. Anything that will keep baby from flying out but not inhibit light/ventilation will work. I saw one lady used an old oven rack(thought that was pretty clever).
Good luck with the new babies!
We have a pen around our brooder made up of wire squares (kind that makes cubes or crates for decoration/storage) and that worked great keeping them in until they started flying out and walking on the dining table! After I walked into the kitchen & found all 5 of them out and walking around I used more of those squares to create a top so they can't fly out. I used zip ties to hold them together (also how we made our rabbit's 5 level cage/mansion). Now they are in the garage (6 1/2 weeks old) until we can get the coop/run finished. Have fun!
I covered the brooder box with a large window screen, that worked fine until they were out in the tractor. THose little buggers are smart and learn quickly. Don't let them know they can fly to the top of the brooder... Breaking bad habits is harder than not learning them at all.
I found rubber matting in the hardware store that has regularly placed holes through it. It is way bigger than the wire squares, about 1 yard square and clips to other squares if that is not big enough. I will be trying that over the brooder shortly.
One of our little EE chicks was eyeing the top of the brooder. As soon as I turned my back she tryed to fly up. Mind you she was only 5 days old. SMACK into the side... but my point is they all try, helps build character.
I've used a screen from a window, and also made a homemade lid out of make-your-own screening from Lowes. When we had a Guinea pig, we used a rubbermaid container and drilled LOTS of wholes in the lid for air vents...but you'll most likely have to make some type of lid if you plan on keeping you little fuzzy butts in the house for a while longer. Look around your house, you'll be surprised what things can be re-made into a lid