You will probably want to be able to close the coop doors at night and move the chickens while contained in the coop early in the morning. It's also a good idea to be able to shut the hens in for selection of specific chickens for culling or other purposes (such as isolating a sick hen).
Build a simple lightweight frame with a cross support for the door, then staple chicken wire to it and to the frame of the tractor on both ends, or all four sides - whatever your design calls for. We used flat metal braces to hold the corners of the door together.
