Most people prefer the look of real wood. Unfortunately, most fences rely on steel posts for durability. You can have both. Typical steel fence posts are round, and set into concrete. Manufacturers have allowed for this by providing simple brackets that allow you to attach wood to round metal posts. The brackets look like a horseshoe with wings. It's quick and easy. When you're finished, the post appears as if it were solid wood.
Place the two horseshoe brackets on the post dividing it into three even segments from top to bottom. Hold the bracket against the post, with the round part centered on the post. Push on the bracket to spread the opening slightly. It will snap around the post with the wings of the bracket extending to the left and right side of the post.
Insert one 1-inch self-tapping screws into the hole on the front or round part of the bracket. Use a drill/driver to drive the screw into the post, securing the bracket to the post. Do both brackets.
Measure the height of the post. Cut two pieces of 1 1/2-by-4 1/2 inch cedar and two pieces of 3/4-by-6 inch cedar to the measurement using the miter saw. Stand one piece of the 1 1/2-by-4 1/2-inch cedar on each side of the post, with the edges against the wings of the bracket.
Screw the two cedar pieces to the brackets from the back side, 6 inches apart on the outside, with 1-1/4 inch screws, a drill/driver and the drilled holes in the bracket.
Place one piece of 3/4-by-6 inch cedar on the front and one piece on the back. Screw the two pieces flush to the front edges of the two existing pieces to complete the box.