I did a little research and found the type of feeder I had seen before. If you're interested, look for "treadle feeder" on BYC (hit advanced search and choose *title only* to limit results. -You probably know this already, but when I finally figured out how to search this way, it was a...
I saw a post where someone had built a feeder that had a cover. The chickens would open it by standing on a pressure plate, and the lid would drop back down when they finished eating and walked away.
If you're handy and enjoy projects... Think it took a bit of time to get the chickens to...