Ours had a small dog kennel type arrangement for many years. Just a flat floor, since they don't perch. Also put in far more ventilation than you think you need!
Make the pop hole at or only slightly above the outside ground level, and make it bigger and wider than for hens because they all tend to rush in and out as a group.
We upgraded our duck housing a few years ago, to "the brick duck house", which is built like, well, a brick duck house! Concrete floor, lifty uppy lid, and one wall made from a wood panel that lifts off to allow cleaning.