I've also seen some made of PVC pipe. Don't know what dia. but i'm thinking 6" dia.

Make the long, straight tube section about 4ft long. Make it so that there is a 90 degree elbow on the bottom and put a pipe cap on it but cut the lower half of the pipe cap off so that it will allow the grain to come out. Also, put a pipe cap on the top of the feeder so the grain doesn't get wet. You can strap this type of feeder to a tree.

I think I saw this online or on somebody's homemade website.....looks pretty interesting, just wish I knew the specifics. Hopefully this will give you a rough guide on how to make one.