Because most domestic ducks do not fly they are vulnerable to predators and need to be penned during the night. Predators can also come from above, so make sure your pen is covered. You can use a cement floor that is relatively easy to clean, or a dirt floor and some use a gravel floor. It is important that you know that some predators can dig into a pen and the floor, if not concrete, should have a layer of hardware cloth covering the floor.  Then and only then fill your pen with dirt or gravel.  If using dirt you should use a bedding (I find straw to be the best). If you use concrete you need to know that concrete alone is too rough for a ducks feet, and the floor must be covered with a thick layer of bedding. It is important that your ducks be able to lay with their abdomen on a dry surface and the bedding must be changed regularly.  If bedding is left wet or soiled a dangerous mold can form and can cause respiratory problems for your ducks.  The walls can be galvanized wire of chain link but at least the bottom three feet must be covered with hardware cloth.  Ducks like to lay against something when they sleep and experience has taught me that Raccoons can and WILL reach in and rip huge wounds in a duck.  Any proper night pen has to have a secure cover, again I recommend hardware cloth.  You can use a standard house type roof or a chain link cover and will need shade and protection from bad or very cold weather.  You have to remember that, at night, Raccoons climb, Fox's dig and Owls fly.  A lot of people allow their ducks to free range during the day.  If you do this you must at least have them in a fenced enclosure that will keep out neighborhood animals like stray dogs.  They are also vulnerable to hawks during the day.  I prefer to use avian netting over the day yard.