I made this bracket so that I could stack my power supply / control boxes for my pair of Maker Select V2 printers. I needed extra desk space. This should also be compatible with all the clones of the Duplicator i3.
Print 2 of the brackets to balance the load. There is .1mm clearance, so you'll get a snug fit on the boxes when you push them on, and it's very stable. I left plenty of space between the two boxes to stop heat from building up, though these don't really got hot in the first place.
Note: If the part comes in too small, scale it to 2540% to convert to MM.