Step 1: Trial and Error
We tried a 180-degree elbow with the edge cut off: the birds were able to eat just fine but they spilled quite a lot, and closing the pipe for waterproofing and rodent-proofing would have required additional engineering. We considered quite a few other variations, but they all had drawbacks; mostly related to spillage and security.
At first the bottom part connected to the "Y" was only three inches long and the birds didn't like that much, so we set it up on a brick and the chickens seemed to like the altitude better, so the final version uses a six-inch length of pipe to place the food where the chickens can easily reach it.
Another way to go (and in response to some reader comments): if you add some kind of plug right at the bottom of the Y, the birds would be able to reach all the food. It will take you more than 3 minutes to assemble, but it would be more efficient. Of course, it's best to use plastic or something else that can be thoroughly cleaned. Most of the plugs I see out there would work, but you'd be back to the height problem (if you're concerned about chicken ergonomics). Easy to fix: just mount the feeder higher. Or: run a long carriage bolt through the base cap (or plug), letting the end stick out and hit the ground, like the spike that sticks out of the bottom of a stand-up bass or cello.
Step 2: Materials, Tools
-Three inch diameter pipe: 20 inches long. Or more. Or less.
-Three inch diameter pipe: 6 inches long
-Three inch diameter pipe: 3 inches long
-45-degree "Y" connector, like this, but this would be quite cool, too.
-Two three-inch PVC caps, like this.
Step 3: Assemble
It only took like three minutes, didn't it?
Let it cure for 24 hours, or until it is no longer stinky.
Actually, you can get by without gluing at all, as long as you're careful when filling and moving: it would be no fun if the bottom cap fell off of a full feeder during transport! But then it would only take like a minute and a half to build, right?
Step 4: Enjoy!
Remove the top cap to fill using a funnel, bungee the feeder to something, and invite the birds!
Place the cap over the opening at night to make the feeder weather- and critter-proof.
UPDATE: To add awesomeness, do what Flodado did: