I do not think that's the solution for me.
So far, I've just designed all interior dimensions of my constructions 0.4mm larger, so that fit inside dimensions.
But apparently there is also a way to adjust your printer so that the outer and inner dimensions fit exactly as designed.
For example, for the 16mm opening for the Hotend, you constructed exactly 16.0mm. I should have made it 16.4mm to make it fit.
So the error is very likely in my settings.