I'm not sure exactly what to use for a duck brooder. For chicks I usually use a plastic storage bin but I'm not sure since the ducks get bigger fast. With the chicks, I usually switch to a large dog crate for about 2 weeks, when the heat lamp is no longer needed (about 5 weeks) but I've heard and read that ducks grow a lot faster than chicks. Could you please post pics of your duck brooder so I can get an idea of what to use (not cardboard as it gets wet and soggy).