Hi there, I haven't seen an elephant leaf planter, however, I've seen bird baths. Perhaps this will give you some inspiration. https://www.hometalk.com/23354802/leaf-casting-in-cement-bird-bath
ideas videos https://www.youtube.com/results?search_query=DIY+elephant+leaf+planters+with+concrete
Begin by making a small hill of sand. Just make sure it's level Make sure it's as big as the leaf. Place the leaf over the sand hill. Next comes concrete mixture over the leaf. Just a half-inch thick layer. fyi: concrete mixture is 1 part concrete and 2 parts sand, mixed together, then add water slowly until you get consistency of frosting. Cover the entire leaf and let it dry for 24 hours. Then start pushing some of the sand out and away from the hill. Eventually you will be able to lift the planter off . You can leave as is or paint the leaf to show all of the veins of the leaf.