I'm sorry that this isn't solid plans but when I made my 3 nest boxes, I bought one sheet of the cheapest plywood available, used loads of free scrap wood that was given to me at the hardware store and kind of eyeballed where I wanted the boxes to go, did some measurements and built them to fit the space.
I built mine right into the exposed studs in the barn wall so whatever the distance of wall studs are, that is how wide they came out. I figured they would have to hop in and then turn sideways and that is exactly how they lay. The floors did not come as far out from the wall as the stud spaces were wide sideways.
The way I got the plywood to butt up to the side pieces was with long, skinny pieces of scrap wood, it could be 1" X 1/2" by whatever length. I would nail say the floor piece to the long, narrow piece (almost similar to baseboards in your home where your floors meet your walls) and then nail that 'baseboard' to the wall piece.
Next I built little roofs that sloped fairly sharply downward so that when the chicken is in the box, it affords her some privacy because it hangs down similar to a house roof and since the nests are built right into the walls, that makes them feel safe as well since nothing can sneak up behind them. It also saved a lot on wood since the barn wall is the back of the little nest house.
Even though I built a good slope to the roofs, I still have chickens climb up there and poop occasionally so I tacked down roof paper to the tops so that it will protect the wood. You can avoid trouble with yours by sloping the roofs down at a sharper angle but I have a heavyset Brahma that I was worried about fitting through the opening under the roof overhang so I don't think I sloped mine any sharper than about 45 degrees. I thought that would be good enough but seems like chickens love to make a liar out of you at times.
