As I have mentioned before, I have an abundance of predators in the neighborhood. In addition to foxes and raccoons, I have once or twice caught bobcats on trail cameras, though never near the chicken tractors. Rabbits are also pretty low on the food chain, and I have found signs of predation in my back yard (piles of fur, and once, a severed rabbit head).
The cages are pretty strong, especially the bottoms. All sides are 14 gauge wire. And the cages fit pretty tightly in the hutch and can be secured with wire. Because of the location of the hutch, it's not really practical to put electrified netting or rope around the perimeter, meaning physical barriers will have to work (I suppose I'll revisit this if I come out to smashed cages and dead rabbits some morning).
The main point of vulnerability is the top of the cages.
I don't think anything could get in, but having a fox pacing on top of the rabbits probably wouldn't make for a very pleasant situation. Eventually I will have water lines running along the top edge of the cages, as well, although that's paused for now because of winter.
My solution is to put some fencing along the top edge of the roof frame:
The distance from the frame to the top of the cages is about 24 inches, so the top row of the 28-inch fencing was clipped off and pieces straightened as before with the cages. With the thinner-gauge wire, straightening it was relatively easy, and it was easily hung from hooks.
I'm using a couple of carabiners to hold the wire to the top of the cages along the sides. I don't think this needs to be as seamless as would be needed if it were on the ground--anything trying to get on top of the cages will be jumping, and hitting a small gap won't be easy. So for now, this looks good.