@dnc I was going to say galvanized steel drip trays. I’d be afraid plastic couldn’t handle the weight of the soil. But you could line the steel bottoms with plastic trays that drain and control the water saturation better.
@dnc I built similar ones many years ago! I used short 5/4x6 planks laid loosely onto cleats at the bottom edge (similar to the 2x4 runners yours appear to have). I then laid in a heavy plastic liner with drain holes about 4 inches above the bottom. I glued tubing into the holes with silicone to direct the overflow out of the box, and I put about 4 inches of a mix of lava rock, lump charcoal, and vermiculite in the bottom to help with drainage, microbial colonization, and nutrient retention.
@dnc Here are the ones I built - they're standing on the ground and are tall enough to be a wall/bench at the edge of this 2-foot-tall high section of deck. The drains go out the back, and I had trellises up the back as a visual break and to support cukes and snap peas.
@dnc Good question -- moisture retention is a concern with a raised deck above ground, depending on your rainfall. Around here in my area (hot, warm climate) I'd likely want to line the bottom with something to retain moisture and prevent loss of water through the bottom (heavy duty plastic or?) but also have adequate drainage so it's not a heavy water logged tub.
@dnc On the other hand, I saw one of those in Hilo, Hawaii (with very high rainfall, tropical) to deliberately provide high drainage in an area too wet to support a normal vegetable garden.
Add comment