Update While I am trying to learn FreeCAD, I made an attempt in recreating this corner tool using it. The FreeCAD project is attached and it has something bonus: parameters. Indeed, you can change the thickness of the plywood in the spreadsheet or the length of the arms and the project will readjust magically ;-).

While making boxes out of wood, PAL or plywood of 18mm thickness, usually the alignment is a bit tricky to do while working alone. This pair of corner alignments may help the process. The corner tools are having marks for 18mm edge and 9mm guides where the hole or screw should be applied. Obviously, you need a pair (top and bottom) for every corner and therefore you need to print as many pairs as you need. In case you need it, the SketchUp Maker 2017 file is also attached. Fill free to use it in case you need to change it for different thicknesses.