I use a chain curtain over the door (made up with surplus from one we fitted to our back door), and it works very well to keep flies out of the coop; it is said to deter predators too BTW. Anyway, if you try it, your hens will take a few days to get the hang of it (let down the chains one or two at a time so they start by going through a big gap and it narrows until a few days later they have to use their heads to push an opening in it).