I find the issue of nest boxes to be over analysed and worried about a little too much. Chickens will happily lay wherever they feel comfortable.
One nesting box for every 2-4 hens with a minimum of 2.
Dark: Keep the box out of any direct sun and try to keep the light levels down. A flap of old jeans material or sacking can shade the nest box.
Size: Nest boxes should be big enough for your hen to stand in comfortably. Usually 12 x 12 x 12 inches is adequate, but building slightly bigger and deeper is usually a good idea unless you are breeding bantams. Smaller for bantams and 6 inches taller for jersey giants.
Nesting material: Many people use either wood shavings (pine usually) or straw. We personally suggest dry wood shavings over straw and dust the box with diatomaceous earth to prevent mites and parasites. Nice hollow in the straw/bedding.
Often a roost bar or step is added in front of the box to give the hen a place to jump to before entering the nest box. Also a "lip" is added to the front of the nesting box to keep the bedding material and/or eggs from being pushed out.
Sloped top: You don't want your chickens to roost (sleep) on top of their nesting boxes (that would be a big mess). We suggest placing a sloping plank across the top of the boxes to tip any roosting hens back onto the coop floor angling the top of the nest box to dissuade hens from being able to roost on or above the nest boxes.
Most people like to raise their nest boxes up off the ground. Just make sure they aren't too high that the hens have a hard time accessing them and that they aren't higher than the roosts or you'll have hens sleeping inside the nest boxes. About 4 to 6 inches high, experiment.
Most will happily use the boxes you provide but sometimes you may need to let the hens choose their own nest boxes. I have a bantam light Sussex that refuses to use anything other than a large bucket as a nest and I use a variety of things as nest boxes including barrels, box shelving from IKEA .