You must understand that I don't keep any fluffy butted "Duel Purpose Chickens" so a nest just 11&1/4 inches wide by 18 inches long works well for hatching eggs. The floor is either a 1 by 12 board 18 inches long or else a floor of 1/2 inch hardware cloth with 4, 12 inch 2x4s at each corner. The back is a 11&1/4 1x12 and the top is a piece of 18x24 inch plywood nailed or screwed onto the four 2x4 vertical frame pieces with the top even in front with the sides. This should leave a nice overhang on the back and both sides to lift the nest with and enough open space below the roof to provide ventilation. A nice addition is a door or else a strong cardboard box to set the nest box into to keep the hen and newly hatched chicks safely confined while you move the nest to the location that you will use for a grow out location. Fully 99.9999 of all hen nest are made without a whit of concern for the poor hen who you are forcing to do all the hard work and make her get buy with what ever contraption the chicken keeper decides is the easiest on his or her poor aching back. Instead think about what is best for your chickens.