The feeder was cobbled together out of bits and pieces of drain pipe I had on hand. Because it was going to be dropped into a hole in the deck I could not just use a standard cap. Instead I had to cement in a ring, a round cut piece of plastic (cut from a cap) and then another ring to hold the plastic in place. A 5/8 in. hole was drilled in the middle of the plastic and a 3/8 x 6 in. eye bolt dropped into the hole.
This creates a pendulum. The white piece of plastic at the end of the "pendulum" is just some scrap that I had laying around. Most anything will work - even a short scrap of 2x2 lumber with a hole drilled through it. The chickens didn't immediately understand what was going on, even when we tapped the pendulum for them. They were quite focused on the food that was dropping, just not on the pendulum itself. Finally we tied a piece of grass on to it, which they went for right away and we were off to the races.
I like this feeder because it only drops what the chickens will eat, leaving very little left over for the vermin. I did not however think of it on my own. While YouTubing one night I happened on to this slick invention here: