First of all, where does one get 2X4's, most are 2X3. That being said, mine have always been happy with ash saplings 2 inches or wider in diameter, that I have cut down in our woods then space far enough apart so they don't poop on each other's heads. I cut a U-shaped piece of 2=by, which I screw into the wall on opposite sides of the coop, lay the branch in the U and use a screw in one end to keep it in place. Then when I have to clean under them all I have to do is remove the screw to get the roost out of my way.
BTY, when they are in the wild do they seek flat branches to roost on?