A little tip for scaring off foxes is to have human hair, that has been cut, stuffed into a pair of tights and hung around the perimiter fence of the chicken run, foxes are put off by the smell of humans.... Around the bottom of the run, or fence around the garden I would bury bricks and broken glass so they cannot dig under it, if they can get over the top, I would raise the height of the fence, do anything you can to make the foxes try someone elses garden for easier access, then let them worry about sorting their garden out.
My allotment is very close to a large field, which is a known hotspot for foxes, I used to always lock my hens up every night, but ever since Christmas time I've let them get on with it and foxes have never been near my garden and my hens are very safe!!!! It's the 2 legged foxes I will have to watch out for!! I have bricks and broken glass around the whole of my garden, the fence is really high and strong, I pee on the fence sometimes to get my scent on it just incase a fox does go wandering past, and just yesterday I planted loads of rasperry and blackberry bushes all around the outside of my garden so they will climb the fence. Not only will it produce fruit for my home made jam, but it will act as a natural deterrant for any1 thinking of climbing into my garden.. I'm sorry that you lost a hen, thats awfull and I hope if you follow any of my tips that your hens will be safe..... Good luck