Man, I work in sheet metal all the time, if you are getting a sharp edge when cutting it to make a feeder something is wrong with the metal shear. But for a homeowner without tools, I would grab a hand size flat rock, sand stone would be best, and grind down those edges. Rasps are for wood, a small metal file would do. some 100 grit sandpaper on a block of wood would work too.
A short chunk of angle iron could be used as a scraper, use the inside corner to de burr the sharp edges. A deburred edge is pretty safe except for a rooster with a protruding comb that can get hung up on an edge. I finally started seaming the front sides of the feed opening instead of deburring them for that reason. That would be cheaper than paying to ship back the feeder.
