We've tractored rabbits for 2 years now, have lost only one or two to something pulling the J-feeders off the front and the rabbits then getting out and disappearing (their cage-mates were caught by us the next morning). Sides are heavy-duty 1/2" hardware cloth, because raccoons can reach through anything coarser. We use panels of "wire decking" for pallet racks as the bottoms and tops. For the bottoms, we take out some of the cross wires so that the holes are around 2" x 6". Tops have feed sacks for shade/weather protection (need to replaced every few months, but luckily we have a limitless supply) covered by more hardware cloth.
We find that pregnant does will definitely dig. Even with the 2" spacing, on rare occasion a freshly weaned kit will squeeze out when the tractor is parked over uneven ground. Still, we're big fans of tractoring!