I love this bird-feeder tutorial from Instructables user me13lake. It’s not flashy (indeed, with a title like “Small simple horizontally mounted squirrel resistant/proof bird feeder that requires no specialist tools to make,” I’m guessing it was written by a scientist or engineer) but it’s well thought-out, easy and inexpensive to make, and it looks like it would work very well. The clear squirrel-guard rotates freely so they can’t get a grip, and can be made from a 2-liter plastic bottle. Or you could try cutting one from a glass bottle.