A garden on wheels is beneficial throughout the growing season. In spring, keep the garden in direct sunlight so the plants get a healthy start. Then in summer when it is too hot, roll the garden to a less scorching spot.
Use pieces of 2x4 in each corner to secure the sides to each other. When all four sides are attached to one another, attach it to the pallet by toe-nailing the 2x4 corners. Flip over the whole thing, and insert a few screws through the bottom of the pallet into the 2x4s.