I get a lot of questions regarding how to keep rabbits safe from foxes. Historically domestic rabbits have been locked up in an area, be it a hutch or a run, around which the fox can encircle them and even if he cannot get inside can truly terrorise his prey.
In the wild the rabbit would hide in too small a hole and the fox would not pursue but accept defeat. I recommend a very safe hiding place for any domestic bunny in any housing set up and also when free in a garden. I also suggest that the rabbit is never trapped in one area however safe. There should be options, ideally 2 exits so he can choose where to go to feel safest. It is a rabbit's nature to flee danger so cul-de-sacs are not good for him.
The favourite hide is often be in the smallest space so his body is surrounded securely. He often chooses the pipe and he needs a section of this where the fox cannot see or smell him. It could be a buried section of pipe (natural, appears that rabbit has gone to ground) or a pipe sleeve (a larger piece of pipe about 2ft long that slides over the existing link to hide the bunny midway through. Make sure even if you let your rabbit have free supervised runs that there is still a well marked (gland scented) hiding spot he knows about.
Hay hutches offer a secure 2 exit bolt hole and top boxes elevate the rabbit up, again with two exits, so they can rest easy that if danger approaches one way they can flee through the other. This is how a rabbit has to think and once we respect their position in the world we can help to build their confidence and help them know they escape capture even if the threat is not real. They are developed to react to any unusual noise or disturbance and being trapped is their worst fear.
The fox reacts to sight of the rabbit either wriggling through something or cornered in a space. If he can see him, especially cowered in a corner, it encourages him. A vanishing rabbit is much more trouble so the pipe exit from the run is perfect to deter the fox from any attempt to dig in. He is intelligent enough to know it won't serve any purpose.
(image below displays the Pipe Sleeve)