Brace yourself! You'll definitely want to pick up a couple of these double 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. Four holes, two 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 10mm long screws (8mm will also work)