This one might be to small for you , but it gives great instructions. Maybe you could just double it.
https://www.backyardchickens.com/web/viewblog.php?id=39717-how-to-build-a-brood-pen
https://www.backyardchickens.com/web/viewblog.php?id=39717-how-to-build-a-brood-pen