I have known for a long time that one of the entry points into the garden for foxes is this gap between the fences of two neighbouring properties. My first attempt at blocking the gap was with wire netting, but the foxes managed to compress it and walk over the top. I then stuck lots of twigs in the soil around the gap, but the foxes still managed to squeeze through. As neither of the fences are mine, I can't attach anything to them, but last night I thought of this solution, which allows me to adjust the length of the horizontal pieces of cane so that they meet the fences. A central post holds the milk bottles in place.
Hope this idea works.