This is the underside of a folding work table, made from extra plywood, scraps, and recycled lumber. All this extra lumber came from my noob mistakes, so I really want to make good use of them.
It’s fastened to the wall by concrete nails, and held by a piano hinge (those things are surprisingly strong!). It has to be sturdy, rigid, but also light, so I used a stressed-skin design (or torsion box). The bottom skin wasn’t attached yet in this photo. No fancy joinery, just glue and nails.
It’s not yet finished, but as it is it is rock solid, thanks to the concrete nails anchoring it to the wall.
-- I still have all ten thumbs