I wouldn't use a heat light in conjunction with a cardboard box
I didn't see this before. No heat lamp! It's a seed grow mat -- like a low-grade heating pad. I put the contraption in the coop tonight, and the chicks seem to like it.

@Morrigan Would you mind keeping track of how much feed you go through for your meaties for me?
I've done it in past years -- I'll try to look up my stats. It's hard to keep track of perfectly because the meat birds and my laying flock get the same food. I know that in past years, when I did all the math -- feed, bedding, cost of chicks + shipping, it all worked about to be about $1.5/pound of meat in the end. Tomorrow, I'll try to find my notes.