I have over 100 birds, so I have several feed stations of the PVC pipe type. Like-ish in this video ...
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I solved my "flow" problems differently than he did because I had the opposite problem. I found that crumbles got stuck in the hopper and didn't move down into the feeder section. I solved this a variety of ways. First, I used a "slower" angle to connect the pieces. Second, I now use pellet feed & whole grains so the feed flows better. Third, I hung the feeders at a downward angle so it flows better.
NOTE ... if you put the holes ON TOP of the curve of the pipe, then the feed doesn't spill from the feeder ... this seems so obvious to me that I have to post the link to the video for a giggle.
I am thinking of a way to attach a larger hopper to our feeders, though with several feeders and access to outdoor forage I don't find filling them to be too much of a chore.
Here is a dumb, unrelated video that shows some of our feeders ...
Second NOTE: I'd avoid the "single hole" PVC pipe feeders with the "Y" fitting at the bottom for feed access ... I've read reports of younger birds getting stuck upside down in those feeders, which didn't end well for the chickens.