I dunno, if you have a 1-2' apron type thing all around your tractor, you will be pretty safe from casual predators. That is, you have a wire mesh panel on each side that either is permanently mounted in the horizontal position (makes tractor a pain to move, ask me how I know) or can flip up out of the way for moves and gets weighted with cinderblocks or pinned with tentpegs. It is a rare predator that will start its digging far enough back from the base of the fence to get under that.
Frankly, few peoples' runs are protected better than this *anyhow*.
Be aware that tractors are hard to winter birds over in, in cold wintery climates (ok in mild areas); they can be hard on your lawn; and it would be hard to build a tractor that *6* hens were comfy in full-time. If they free-range all day every day all year, no problem, but otherwise, they are going to end up awfully squished in (unless you use a very large, broiler-production-type tractor design, which has its own problems) and crowding is just *asking* for trouble.
Just sayin',
Pat