I built this cedar planter over 20 years ago and I must have done something right because it hasn't rotted out or fallen apart yet. I over-built it, using 2x4s to hold it all up. And I coated it generously with an outdoor grade clear-coat. At some point I made it deeper and I can tell I didn't put the same care into the extension because it's breaking down much faster than the original wood.
My plan is to add a water reservoir to the bottom and turn it into a wicking box: a self-watering container. And while I'm at it I want to scrape and sand down the outside of it and apply a few more layers of wood finish.
I was surprised by how much potting soil this thing holds. I filled two large garbage bags, which I plan to put back into the container when it's finished. The bottom of the box was lined with a yellow garbage bag and that must have helped protect the coated 1/2 inch plywood bottom of the box. Even the ends of the 2x4s that poke up into the growing area were still solid. Lots of clear coating and silicon sealed joints kept if protected over the years.
When I attached the second layer of cedar boards to the original container I used some 2x2 blocks and nails to hold it together. Those were not treated in any way and years of being under the potting soil have rotted them. I'll have to fix that up.
Something I found interesting was the moisture distribution in the box. The potting soil on the side that sits along the outside of the balcony, where it is more likely to be kept moist by rain, was nice and damp all the way to the bottom of the box. But the potting soil in the side of the box away from the edge of the balcony that doesn't receive as much rain was bone dry, all the way down. I would have thought the moisture in one side of the box would be pulled into the side that was drier but I guess not.