Been there, done that...until I finally found a squirrel proof feeder. It is metal with a spring adjustable feeder bar. The birds can hop on the bar to feed, but if a squirrel tries, his weight drops the bar and closes off the seed. You can probably see it in one of the pics of my deck.
I've seen those. They start at about $30. I have other priorities before I can invest that much in bird feeders.