I live above a business and am going to make a garden with containers and these two wooden boxes that were going to be dumped. I plan to plant all my greens in these two boxes. They have not been painted, primed ect. What should I put on the bottom so the soil does not come out between the wood pieces? I hope this makes sense. I still plan on drilling holes for drainage, but I feel if I don't put something over the gaps between the wood I think I will be losing a lot of soil. This is my first time doing this, so I feel a little lost. Thanks.