Used a 2x10 cut 12 inches long for the sides and screwed a 12" x 15" piece of particle board to the bottom, with 1 1/2" sticking out on each end. Then screwed a 2x10 cut 15 inches long for the ends so they could be removed when the concrete cured. You end up with a box 12 inches square on the inside with a particle board bottom. All the lumber came from a residential construction site. Real easy to make and can be used over and over. In the process of building about 10 more so we can build about 20 at a time.