Recess the floor inch up from the bottom to further prevent rotting caused by moisture. Don't put perches on any bird house. The only birds that prefer them are starlings and house sparrows. Most birds are territorial and protect the area around their nest. Goldeneye, mergansers and wood ducks are not territorial, so you can place their houses closer together.
Wood is the best material to use.
Avoid pressure-treated lumber because when it gets wet it can give off vapors that are poisonous to birds. Preservatives, such as paint or stain, can be used on the outside of the box, especially the back, but not on the inside. Avoid using creosote as a preservative. Do not use tin cans, milk cartons or metal for nest boxes. They can overheat and kill the eggs and young birds.