For aluminum profile 30x30, using M6 screws.
Added a version with cutouts for the washers: 57_cm_Right_Angle_Bracket Version 2-2.stl
Added a variant with a reduced length of the guides: 57_cm_Right_Angle_Bracket Version 2-1.stl
04/24/2016 A little changes at corner: 57_cm_Right_Angle_Bracket_Version_2.stl
Last variant is the version with cutouts for the washers and reduced length of the guides.