Amy, have you checked out the BYC Brooder section? Maybe it has something that might work for you, or that you can adapt to a larger scale. (Be sure to keep scrolling down to the bottom - lots of ads in between.)
https://www.backyardchickens.com/chicken-coop-brooder.html
~Gail
https://www.backyardchickens.com/chicken-coop-brooder.html
~Gail