For the specula holder the sketches that you can see in the screenshots are the one from the file already in the repository and my modification that IMO looks cleaner and more easily understandable if needs to be modified. The final body is same.
For the head lock I used eliptical sectors (which were used in the tinkercad sources) in the sketches instead of complex b-splines. Also used PartDesign for cleaner tree and easy manipulation.
You can do whatever you want
For the Head lock there should be some common geometry description for the mating profile since it needs to fit several parts. After all parts are made those should be checked for corresponding geometry and if created from simple sketches it would be easy to make them fit.