There are so many great tractor type coop designs but all share the risk of predators pushing under or digging under the perimeter of the run section. If you had spikes (similar to my awful childhood vacation tent memories- ha) that you could stick through the hardware cloth, into the ground -creating a sharp spike deterrent to diggers- could it work? The spikes would be about 10 inches, slammed down with a sledge hammer or regular one. (stop laughing at my drawing) It could be double spiked or just shoved into the ground with the short end looping the pen. At it's worst, it would hold the coop down a bit- but if you had a one in every square it would be like sunken hardware cloth. It would also let your tractor be mobile.