Not to worried about anything opening the door. It may be hard for you to see but it is smooth metal and has quite a bit of weight. There are no cracks where a critter would be are able to get any kind of grip to open the door. Valid point though. I was not aware of any other designs like...
Really did not take that long to tweak. To start I had multiple levels on the moving roost so all the birds would need to be on it. I found that to be a little clumsy so I came up with the 2 stationary roosts and the single moving roost. Seems to be working really well now.
From what I've seen I believe it will be more reliable then anything with timers or power. Simple in design and nothing to go wrong. The birds seem to be very consistent in there habits. A timer has no idea what the chickens are doing. This functions based on the natural habits of the chickens...