My husband has had good luck using johnny stuart preditor tapes. They Have a pleeting chicken but what worked best was using the yellow shafted woodpecker tape. He used a cassette player with a remote speaker camofloged in some leaves. He sat quietly compleatly camoed out. You must hit them the first time or you have just educated your fox and it will not fall for it again. Use a shot gun buckshot prefered, less chance of error and you may call a coyote in. He has had bobcat come in and sit listening within 15 feet of him.
We have 3 guardian dogs loose and patroling during the day. This is how i free range.
I still have lost 6 hens this year, i think to a very sneeky bob cat. Our game cams have caught him and several coyote on camera. No fox pictures this year. I think the coyotes have killed them off or pushed them out.
Killing your current fox will not be a long term fix. Every eats chicken. When you eradicate this one , more will move in. Keeping chickens is a constant battle.
If i were you i would start planning to put a run up. Just for these times when you need to secure them during the day. I have a run that i can put in use when i go on vacation for example. Then my chore person has less to worry about. And i worry less also.
Good luck!