Quote:
People have used lots of things but a piece of wire fence is the easiest because it is easily formed into a cave and the height and shape is easily adjusted by using the highly technical MHP term "smooshing"
A left over (and that may be your problem, no left over wire??) 12" deep and 24" wide (the open side) for a 12"x24" Sunbeam Xpressheat pad works quite well. If you use 1/2" hardware cloth (which you may have when you finish your coop and run) works but you'll need to "reinforce" it so it doesn't sag down when the chicks start sitting on top. Chickens LOVE to sit on top of things. Same with chicken wire if you are using that (but it shouldn't be used on your coop, it is NOT predator proof). Other options people sometimes have around are 1x2, 2x3 or 2x4 welded wire fencing.
BUT others have successfully used a piece of linen or pantry shelf or other flat racks of various nature. The can't be smooshed so you need to come up with another way to get them at the right height as the chicks grow. But they also don't need to be "reinforced" since they will hold up a ton of chicks without sagging.
Look back through the thread, you don't have to read it all, just look at the pictures.