A basic nest box need be nothing more than a simple wooden box that provides a dry, dark place in which the hen can lay eggs. You can construct the box with a bottom, top and sides made from 1/2-inch plywood and one-by-two battens along the inside corners; assemble the box by screwing through the plywood and into the battens with 1-inch wood screws, drilling pilot holes first to prevent the wood from splitting.
The nest box needs to be big enough for the hen to stand in comfortably. A box for bantams and other small breeds may be as small as 10 inches wide and 10 inches deep, but a typical box should be at least 12 inches wide and 12 inches deep. Boxes should be at least 12 inches high in the interior; boxes for large breeds should be 14 inches high. The opening at the front of the box should be at least 12 inches high.
Lips and Roosts
A 4-inch-high lip at the front of the box will keep bedding material inside the box, and will prevent eggs from being pushed out of the box. The lip will also provide a place for the hen to land as she enters and leaves the box. In addition to the lip at the front of the opening, you can attach a roost bar that projects from the front of the box to give the hen a perch on which to land or rest outside the box.
The roof of the box should be sloped to discourage hens from roosting on top of the box and making a mess. A steeply sloped roof of as much as 45 degrees won't give the hen a comfortable place to perch, and she'll seek out some other place to spend the night.
Mounting the Box
Mounting the box on a wall or other vertical surface well off the ground will help to keep predators away from the eggs. If the box is too high, however, the hen may have a hard time reaching it, and if the box is higher than nearby roosts, the hen will be tempted to roost in the box. A mounting height of 3 to 4 feet will typically provide both protection from predators and easy access for the hen. Make the box cozy and safe for the eggs by lining it with straw, wood shavings, or in a pinch, shredded paper.