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The store that sells the PVC tube also sells the caps; I use one every night.
You may have daytime rats stealing feed. I caught them on my trailcam, with chickens all around them, pulling themselves up to the opening & chowing down.
If your birds are flipping feed out, put an extension in the "Y", cut from the end of the upright tube, so they have to reach further inside. This is how mine was before I put the extension in, with feed all over the ground.
