Adafruit Aluminum Extrusion Corner Brace Support (for 20x20) [ADA1155]
Brace yourself! You'll definitely want to pick up a couple of these corner brace supports when working on a project with 20x20 extrusion. They're nice and solid, made of cast aluminum so they are stronger than extruded corner supports. Two holes, one in each corner allow you to easily connect two pieces at a 90 degree angle.
Comes as just one piece, you'll also need T-nuts and 8mm long screws (10mm are too long!)
Aluminum Extrusion Corner Brace Support (for 20x20)