The chicken tractor we built was an A-frame, but you can build a box or something with a shed-type (slanted) roof. It's up to you. Use a table saw to rip 2x4 lumber into thinner pieces to build the frame, and deck screws to fasten it together. If you use nails, it will come apart with multiple moves.
The two additional bottom pieces, or stringers, provide greater stability to the bottom frame of the chicken tractor.
As you'll see in later photos, we added collar ties across our A-frame structure to add stability. Then we built the end framing - simply three pieces of wood that form a frame for a door in each end.