Plastic Planter Boxes are the cheapest of planter boxes which you can buy. These are very easy to maintain also.
Use a liquid detergent of good quality. Pour a few drops detergent on a soft cloth and apply gently on the Surface and wash off the same with plain water after 5 seconds and dry.
You can use a good quality wax polish. Apply a thin layer of Wax polish with a soft cloth and allow the same to dry then polish with a cloth to get a lustrous shine.