Amazon.com: boeray 3-Way End Corner Bracket Connector with Mounting Screws for Aluminum Extrusion Profile 2020 Series Pack of 4 : Industrial & Scientific
Aluminum Extrusion Double Corner Brace Support (for 20x20) : ID 1259 : $1.95 : Adafruit Industries, Unique & fun DIY electronics and kits
Aluminum Corner Extrusion Double Sided for 1/2" Panels - Star Case