I'd go tongue and groove, use 4x4 posts (grooved) and 2x6 redwood (tounge). Thin materials are going to bulge from weight and the pressure of expanding soil when you water. The screen on the bottom will prevent underground attacks on those lucious roots and crops. Keep the bottom open for drainage, maybe even add a layer of river rock an inch or two deep on the bottom to help drainage. Four feet square is about as large as I'd make it, but I might make it 24 inches high as a back saver. That's a lot of soil, which I'd mix up to match what I was growing.
For flowers, I'd raise the bottom up since you don't need deep roots. Use a layer of window screen and the rocks over the critter shield. And I'd put a 2x8 across the edges on two sides so you can sit there. Definitely would assemble all using coated deck screws, including through the posts and tongues. I would probably use some sort of rubberized coating on the inside corners to help keep grit out of the joints.
I live in the desert, and summer would require a sun shield for many veggies (tomatos for example) so I'd add a way to put a pvc frame on during summer. I used to have a couple of these, but they inevitably deteriorate and mine are now long gone. Wife doesn't want to garden anymore.
The more I do, the less I accomplish.
Last edited by DesertRatTom; 03-01-2018 at 12:24 PM.