Wood ducks and Mandarins use the same type of nest box.  Typical dimensions are 12 X 12 X 24 inches high with an entrance hole of at least 3 X 4 inches.  Inside the box you need to provide a way for the ducks using it to be able to climb out, this can be done by tacking hardware cloth up the inside front of the box or you can cut grooves in the wood also pictured below.  The bottom has a few ¼ inch holes drilled for drainage and inside is placed about four inches of wood shavings for bedding.  The top or side should be hinged to make inspecting and cleaning easy. When placing a nest box in the wild, try to avoid using trees as it will be an easy target for a predator such as a raccoon.  A nesting box fixed to a wood post with a sheet metal type cone as a predator guard is best for the wild.  The cone is the same type used to prevent squirrels from climbing up to bird feeders only larger, it should be placed eight to twelve inches below the bottom of the box.  The number of nest boxes is important, when the number or hens exceeds the number of nesting sites a condition known as “nest dumping” occurs.  Hens will typically lay their eggs in a nest that is already being used and the result will be too many eggs and the hen will abandon the nest.  Try to provide enough nest boxes to avoid the possibility of nest dumping.  If your birds are pinioned remember to provide a ramp for them to access the nest.  Like I mentioned earlier, if you can find an old hollowed out tree limb you can add a top and bottom to it, cut out a hole and have a natural looking nesting area.