People have actually been doing it for years. My uncle has a feeder behind his house in WI for watching purposes, and after the season he puts a 2x4 up on each side. So when the buck is done feeding he'll pull his head up and knock it against the 2x4. If they’re ready to come off they do. There’s more than enough clearance for the buck to take his head and antlers out. He just took advantage of the bucks lifting their heads when their done eating.