I followed along with you in class for the first half of the tutorial and did the second half on my own just following the guide.
For the second piece I wanted to attach it to the bottom of my monitor, since that made the most sense for the setup of my desk. The top open space of the piece is 14 mm wide to attach to the monitor. It's 11 mm tall to fit the majority of the bezel. The bottom opening is 4 mm wide, which is slightly smaller than the width of my cable, so the cable can easily be worked in with a little pressure but will not be able to fall out if left alone.
The last two parts could be put together and easily taken apart, but when put together would not come apart if held upside down. The cross on the first piece is 4 mm tall and the pocket on the second piece is 4 mm deep.
FreeCAD can be a pain but there's a nice simplicity and precision about it.
