To make your own cheap, large outdoor planter, use concrete and two molds. The molds can be of any size you desire; however, one mold must be slightly larger than the other. As of 2015, the price of concrete is approximately $3.35 to $5.25 per square foot, depending on the complexity of the concrete. For this project, you need concrete mix, cooking oil, a paintbrush, water and two containers, with one small enough to fit inside the other.
To make a large outdoor planter, first prepare the concrete mixture. Next, use the paintbrush to coat the inside of the larger mold and the outside of the smaller mold with cooking oil. Fill the larger mold with concrete mixture to about one inch from the top. Shake the mold to eliminate air bubbles and level the surface. Finally, press the small mold into the large mold.
When you make a large outdoor planter, leave adequate thickness on the bottom of the planter to support the weight of any soil or plants you plan to add. Let the concrete set for 24 to 48 hours, then remove the mold. For best results, put a heavy weight, such as a rock, in the interior mold as the concrete sets. Smooth any rough edges with a file or stone.