One wall brace to keep the tubs from moving back towards the trike fenders, and the other to keep them from going forward. Cleats on the floor to keep the bottoms from moving around. So the tubs sit down between the cleats, and the second top layer of them is stacked on. Then ropes are ran from the bottom, up and over the top tub, and back down. Likewise another rope goes around the tubs away from the walls. So the tubs can't really go anywhere.