I am sorry about your loss. have been there myself, it's tough to rebuild a flock after such losses.
Fox are called crepuscular hunters (cats are too), meaning they most actively hunt the few hours surrounding sunrise and sunset. I learned this after losing 7 chickens is 1 morning to a fox. Thing is, I learned, that if you have 1 fox, there are more. If you kill one and continue to free range unattended, you will likely lose more. Also, Call for your chickens, we had 1 return the next morning. Use a familiar sound. We have these critter alarms we use at night to deter predators (and lots of other stuff), my husband sounded the alarm the next morning and one came running home.
I only free range when I am actively watching my chickens, I learned that chickens in runs are better than dead chickens. I had a perfectly healthy flock that were all due to be at peak egg production right now, lost to my stupidity. I don't blame anyone who does free range despite the dangers, but I can't handle losing my pets to such things. I live in the woods and those things will continue to happen if I let my guard down again. Good luck