For adults, I use this.

Bottom half of a large plastic dog crate, sawdust pellets in the bottom, and a flat-bottomed stew pot, with straight sides.
For ducklings, I would put a chick waterer in there instead of the stew pot, with a head washer (margarine tub with a head-sized hole cut in the lid. Watch closely for 15 to 20 minutes after putting it in the brooder to make sure they don't get into trouble with it.
Bottom half of a large plastic dog crate, sawdust pellets in the bottom, and a flat-bottomed stew pot, with straight sides.
For ducklings, I would put a chick waterer in there instead of the stew pot, with a head washer (margarine tub with a head-sized hole cut in the lid. Watch closely for 15 to 20 minutes after putting it in the brooder to make sure they don't get into trouble with it.