I agree with the others who suggest hiding places. Boxes, or piles of branches, or hay bales, or solid boards along the base of the run... lots of options.
Another option, either along with hiding places or instead of them, is to have someone bring a dog to visit regularly, to get the chickens used to being safe IN the run even when scary things are outside it. Also consider lawnmower, weed whacker, children running and yelling--anything you can control, so the hens get a bit upset but not terrified enough to hurt themselves. Gradually increase what they can tolerate.
I know from personal experience that chickens tend to panic the first time a lawnmower is used near their run, but after a while they just don't care. A predator will probably be scarier than the lawnmower, but it's surprising how many things chickens can get used to in time.
sorry for the circumstances. You might try providing hiding places in the run, but their instinct will be to flee at the approach of the flock. I would go about eliminating the fox as it will very likely return.
, hope they get better. Like what @sourland said, places for them to run and hide would be great. You could maybe put a plant in the run for them to hide under, or hang a piece of plywood up high for them to fly on whenever the feel threatened.
