I've been planning to make a post on this a while back, but... OH LOOK! CHICKENS! It's not something I had planned on doing at all, other than having a pulley to open the door from the outside. The door is fairly heavy, and slides past the bottom of the opening to reduce drafts. After testing the door to see how difficult to open from the outside, I was fairly confident with its security, until I used a screw to simulate claws and lifted, which gave it that extra little traction to pull it up. I could add more weight, but wasn't too keen on making it any heavier. I could design a latch that fits over the door to lock it in place, but that would require me to go back into the run to secure it. It didn't need to be complicated, just something that I could rig to the main pulley line that would also keep the door down. There's a better solution that is far more simple and effective than the method I came up with available here on BYC (https://www.backyardchickens.com/forum/viewtopic.php?pid=2467744). I couldn't find a suitable bar for this set up so I decided to use what I had available. The basic setup is simple. You have the main line (clothesline) set to open and close the door. You have a secondary line (nylon cord) connected to the main line that gets pulled/released along with the main line. The secondary line is kept taught by a counterweight. The locking block (2x2) sits in the middle of the secondary line. When the main line gets pulled, the secondary line pulls back the 2x2, then the door pulls open. When the main line is released, the door closes and the counterweight draws the 2x2 back over the door. It was installed relatively quick, and actually worked better than expected right off the bat. There are some areas that need to be adjusted/fine tuned to get it right, such as the counterweight (and stopper), the connection point of the main line and secondary line, and most importantly, how much slack there is on the main line. Here's a shot from outside the coop. You can see the pulley on the inside, and a weight on the main line to create slack. The slack assures the secondary line gets pulled in time before the main line gets taught enough to open the door. Here is the locking 2x2 in place over the top of the pop door. The 2x2 sits in a track to keep the block steady. The secondary line (leading off the main) goes through 3 eye hooks, then connects to an eye hook on one end of the block. When the main line is pulled, the 2x2 slides to the left. The secondary line connecting to the main line needs to be some kind of slipknot to allow adjustment. I have a coated wire clamp thing (silver) to keep it in the position that I want. The brass quick release hose thing was added to create slack. Here is the counterweight made out of a 2x2 and a heavy industrial hook thingy that was in the garage. The secondary line goes through 3 more eye hooks and connects to the eye hook that is on the top of the block. Here's the slider track. From left to right: 2x4, 2x2, 1/4" hardboard, 1x3. Initially this track was not painted, so the hardboard had the smooth side facing inward to help it glide. Painting it (with primer) was a bad idea as it didn't slide at ALL. Finished it with semi gloss and it still wasn't all that great, mainly because the soft furry side top of the 2x2 track now became hard spikes when painted. I used various materials to rub the rough areas down and increased the counterweight and the problem is gone. Top view of track. Here's the door raised, counterweight side. Here's the door raised, slider side. You can see the silver thing on the line that's used to clamp coated wire together. I loop the secondary line around the screw and tighten it down. This is how far the counterweight has to go before it stops, and holds the 2x2 block over the door about 1-2". There you have it. It's not pretty, but it gets the job done. I also have an external hinged pop door due to strong winds we sometimes get.