In this Belgian ;(Dutch) article is a drawing of a safe run. The article can be translated with google.
https://www.natuurpunt.be/pagina/bescherm-je-kippen-tegen-de-vos
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PS: the translation by google:
The work
Stretch a sturdy iron mesh on the outside of the posts
- 1.45mm thick
- mesh size max. 3 x 3 cm
Make your fence at least 180 cm high so that the fox cannot jump over it.
Fold the top 40 cm outwards at an angle of 30°
Equipped with two electric fences on the outside: at the bottom at a height of 20 cm and at the top at a height of 160 cm or
Span the entire run with a sturdy net or (even better) with the same mesh as the fence
Place a row of heavy tiles at least 40 cm wide on the outside of the run so that the fox cannot dig under the wire or
Dig the mesh 50 cm deep at an angle of 45° outwards or straight into the ground.
Other tips
Light up the area where your chickens stay
Keep a dog on your property (fox is very afraid of dogs)
If you do not want a cage or run, you can also leave your animals completely free so that they have more escape options.