I'd just keep the cat confined. I have a 25lb cat (not overweight, just HUGE) who is very predatory. So, since we got the chickens, he is not allowed outside during the day. Period. I thought he'd make a fuss since he was used to going out at will, but he adjusted really quickly and now seems to know he can't go out until they are locked in the coop at night. In fact, I quite often see him sitting in the basement window watching me give them their mash and get them settled in the coop for the night and by the time I head back inside, he is waiting at the door for his "turn" to go outside.