I'm so sorry about your bunnies.
We have urban foxes here and although I've never seen them in my garden, I know they must be about. I have double meshed everything and have a 12ft x 12ft aviary as I can't risk any sort if free ranging as it would only take a fox a second to pop over the 8ft hedges sadly. I do have puppy pens that I use to extend the aviary and give them some extra space when I am cleaning then out, but I don't even pop to the loo without shutting them in again. I do feel sad that my bunnies live such a restricted life, but I feel the risk from foxes here is just too high.
It's true that foxes can scare rabbits to death. 
my vet (a rabbit specialist) feels that double meshing will help as there's a gap between the layers of mesh which means the fox can't get so close to the bunnies if they are laying next to the mesh. Having said that, we have dogs and my bunnies aren't scared of them, although the dogs don't harass them, so they may feel differently about a fox.
You can mesh the floor of a run and turf over it, but that obviously means you can't move the run. Runaround do some good and fairly fox proof runs with mesh floors or skirts, but they are very pricey. The tunnels are excellent though (I have them for playing in and connecting my kennel/run to my aviary) and I have heard of rabbits hiding in them to escape foxes. The lady at runaround is very good with advice and happy to chat on the phone. Their customer service is outstanding.
It sounds like you have a wonderful home to offer some more bunnies. Please don't feel guilty. I had two hens taken by a fox after forgetting to shut the henhouse door one night. I did feel dreadful, beyond dreadful, but we are all only human and foxes can be very determined.