My wife and I built a cold brood box (aka Mother Hen Box). We got the inspiration from chookyinoz in the Backyard Poultry Info Centre.
This is really simple to make and will keep your baby chicks warm.
You start of with a box that can cover your baby chicks comfortable with quite a bit of headroom. Our box is about 30 cm tall. This box make sure you create an entry that the chicks can walk in and out of comfortably. In our case we just cut of one of the flaps of the lid and taped the other three flaps together so they extend three of the four walls.
Now you get yourself an old towel and cut it into strips about 15 cm wide and the height of your box.
Each of these strips you cut further strips into about 1 cm wide but not all the way across.
Now you roll these strips of strips along the uncut edge and push them through holes you've punched through the top of the box so that the strips dangle down from the top.
Once you are all done, you introduce the box to your baby chicks who will be nice and cosy warm from now on and will love this box!
The last picture shows how the box looks from underneath when held up. As you can see there is plenty of room between the strips yet they are all touching.