This is a corner bracket for Misumi/OpenBean Aluminum extrusions. Designed in OpenScad, this can switch between 1515 and 2020 easily and create a plate for those sizes. With a little tweaking it could be extended to larger sizes easily. Use the customizer to change the extrusion width and hole sizes. This will be part of an instructable on how to build the box displayed.
In order to build the tall box that is pictured, you will need to obtain the following items from Misumi (or similar items from another company), the following items are Misumi part numbers.
2 X HFS5-2020-454 (Door assembly)
2 X HFS5-2020-960 (Door assembly)
3 X HHPFRN5-SET (Plastic channel)
4 X HFS5-2020-1000 (Box Assembly)
3 X HSCLS5-B-SET5 (Door assembly)
2 X HNKK5-5 (Box/Door Assembly)
4 X HFS5-2020-460 (Box Assembly)
4 HFS5-2020-500 (Box Assembly)
120 X M5 x 10mm Socket head screws
5 X Laminated 1/4 inch board or other material for the walls
Since these are support brackets, I would highly suggest that you use solid infill on these parts to prevent any issues.