Holes don't go all the way through so that the hinge pin doesn't fall out.
I wanted at least one layer to be skipped. If you make the gap exactly a layer, and the to and bottom layer happen to coincide with sliced height, it might not leave a gap. I guess I could have calculated the height of the gap so that it straddled the sliced height, but you would need to know other slicer parameters such as the first layer height.
For the broom diameter, you can make it exact, or perhaps add 0.2mm. Doesn't really matter. To get the perfect fit, it may depend on how well calibrated your printer is.