This is a Tri-Corner mount for 8020 aluminum extrusion. Please use caution when printing this model, if it is being used to support large piece of aluminum extrusion. My recommendation would be to increase the infill, wall, and top/bottom layer count in order to support the weight of the metal. Use a 5/16-18 tap to tap the ends of your aluminum extrusion, and 5/16-18 bolts to attach the corner bracket to three pieces. Can be used to have a nice end cap for only two pieces of extrusion, but will not be as weight bearing as a 3 corner configuration. 

5/16-18 button head bolts will work best for clearance between fasteners, but socket head screws can be used in a pinch. 

The supports are slightly a pain, but should break away with flush cutters.