For *meat chickens*, which is what the original poster was asking about, you want a different kind of tractor than what most people are going to be using for their laying and show and pet birds. (For a whole variety of good design reasons).
I'd suggest browsing tractor threads in the Meat Birds Etc section of BYC, as there are lots of broiler-specific examples there that you can choose from or get ideas from.
The most common way to keep predators out is probably just having the tractor enclosed within a fairly predatorproof electric fence, either specially for the chickens or because it contains other livestock as well.
Failing that, most people just trust to luck and the fact that the chickens won't be in there all that many weeks. You *can* make a wire apron to lie on the ground all around/beyond the tractor edges, but that will be a big nuisance when you're moving the tractor once or twice a day.
You cannot have a mesh floor in a tractor of this type, as it would prevent you from bieng able to move it.
Good luck, have fun,
Pat