Wire bottom doesn't really work (can't move the tractor, b/c chickens can't walk along). Also prevents scratching.
The normal solution -- this may be what ridgerunner is describing?? -- is to have an 'apron' or 'skirt' (whatever you want to call it) of heavy gauge wire mesh laying flat on the ground all around the edges of the tractor. You can use whatever you used for the tractor itself, or anything else heavy-gauge up to about 2x4" mesh size. You'll want at LEAST 6" width to the apron, more is better, especially with significant predator problems; for wider widths you may want to have it hinged so as to flip up for easier tractor moves. (If it is on the narrow side, it can jsut be a physical extension of the mesh on the sides of the tractor).
I wouldn't personally suggest using any wood on it, though, as that not only adds to weight and cumbersomeness, it also telegraphs to the predators where they might ought to dig beyond. Tentpegs or bricks work well for me to keep the edges of the skirt on the ground.
Good luck, have fun,
Pat