Measure and cut the 2-by-2-inch post into four equal pieces of 12 inches each, making sure that the ends are square. Sand off any rough edges.
Measure and cut the 1-by-1-inch post into eight equal pieces of 12 inches each. Sand off any rough edges.
Nail two 2-inch posts and two 1-inch posts together to form a square, with the 1-inch posts on the top and bottom of the square and the 2-inch posts on the sides. Use a steel square to make sure that all the right angles are true. Construct another square in the same way.
Use the four remaining 1-inch posts to join the two squares together into a cube. The four larger posts should become the four corners of the planter, and the smaller posts will form the support struts on the top and bottom.
Measure and cut the plywood into five 12-inch squares. Nail to the inside of the framework to make a solid cube with an open top.
Turn the planter upside down and screw a castor to the middle of each one of the corner posts. Finish the planter by painting it with exterior paint. Allow to dry before using.