Most garden centers sell a variety of box containers, along with the appropriate hardware to mount them. Other bracket styles make railing mounting a breeze. But again, be sure the brackets are right for the container. Even the lightest boxes can pile on the pounds with the addition of soil, plants, and water. If you have an older home designed with window boxes in mind, and weight isn't an issue, stone, metal, and terra cotta boxes are all handsome choices.
Wood is lighter, of course.
When constructing a box display, follow the same steps you would for any potted plants. Use a good soil mixture, packed over pebbles and stones to insure good drainage. Sun and shade plants don't mix. Plants should be monitored frequently, and dead or dying plants be removed as soon as possible, since they are a magnet for molds and pests , says Radocha. Then add your soil and plants, as you would with any other container.