Electric fencing is a good compromise between total free ranging and keeping them in a small run.We have decided to build a new coop and use the electric poultry fencing to be part of their run.
I agree with the above poster. This fox will be back, so you need to keep your chickens under heightened security until the problem is solved. Keep in mind that even if you trap or kill this one fox, if they are in the area, others may take over it's territory. Free-ranging is always going to carry some risks. You have to decide if that is acceptable to you or not. There's no right answer here, it's just a matter of how you balance safety versus letting your chickens have more freedom.
Very sorry for your two girls. It's always hard to lose a pet.