It can be a challenge to think about how to make your own silicone mold for a cement planter- or for anything, because you have to think of the positive and negative space in reverse.
Step 1. Trim The Plastic Board For The Box To make the box for silicone mold, cut the corrugated plastic board to size, using a craft or X-acto knife.
I then added ½” to each side measurement, as well as the top. So with the ½ added, I added up the length of each side, which became 14”. Then I marked each of the sides at 3 ½”.