The cubes for the cube case are just corner pieces that the boards screw into, they should be 10mm on each side. You need to print 8 of those for a Cube. You need 4 washers for each board that you want to attach to the rack case. I will make this clearer with some proper instructions. In the meantime, I will amend the post to include quantities and some build tips.

I’ve added .stl files, so that may fix any sizing issues you might be having.