If you often print boxes and other similar things you may want to know what gap size you prefere for this thing. This thing has six cubes with different gap size to show you what you need.
Cubes are markes (0.1-0.6) that's size of the two side gap between box and holder.
For example, 0.1 cube dimansions is 19.9x19.9 and the size of the recess of the holder is 20x20, that means that the gap will be 0.1mm.
Print it with you prefered settings. If you printer\settings is not good enough 0.1 and 0.2 cubes may not fet.
Just be shure that you remove retraction dots on the walls before joining parts together