Lots of things would work to rig up for a bigger area. A bigger box, a dog crate (taken apart, only use one side), an area in a shed or corner of the garage. Remember too, soon they will be able to jump and fly up and out! You may need to think taller or a mesh cover or some sort.
They need the warmth from the heat lamp or at least make sure the air temp is the right temp for their current needs. 95 degrees now, decreasing about 5 degrees each week until they have their feathers. If the outside air temp is what they need, and you can provide a dry, secure area, they could go outside for an outing. I must stress secure! They can move fast and so can dogs, cats, hawks, etc. Gotta protect the babies.
You will be fine, just relax and enjoy the chickies.
Oh, I hate it when I do typos.
I meant crowded, not crowed in my first post.