If you keep laying hens, you'll need nest boxes where they can lay eggs. These can be simple and built into the frame of the tractor, and lined with hardware cloth.
For the lower nest box, we built a simple box out of 2x2 wood and added a piece across the middle for stability, then stapled hardware cloth onto the bottom. We laid this over the stringers that make the roosts and screwed it to the end framing and the roosts.
We built 4- to 6-inch high sides of plywood, screwed directly to the frame of the nest boxes. Make sure roosts and nest boxes are fastened to the structure for ease in moving.
To use recycled materials you may already have, you could attach plastic tubs or recycled wooden crates to the frame of a chicken tractor with screws.
Aim for one nest box or one square foot of community nest space per 4-5 hens. Nest boxes should be at least two feet off the ground.