I agree with the pulletshut door...it's great. You can get different options too, solar charge, or electric..and either light sensor or time program. I'm getting another one for the broody side. Its recommended to protect it from freezing rain/snow, as anything mechanical. It is also pressure sensitive, so if a hen is standing there it won't squash her. No predators can open that clamshell.
I think that if someone had the knowledge and experience to build an autodoor, they might be able to save some money, but when you calculate all the extra expenses, it seems very reasonable cost wise. Like, welding material, if you have the machines, trips to round everything up, labor time itself. Return trips to the hardware store, time spent searching for parts. Time to make the thing...then there's the programming...like I said, if you have the expertise, which I don't, I consider that I am paying for that too. Well worth it and very reliable.