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Have you considered, perhaps, trying the FreeCAD 1.0 release candidate? It should be very quick to make this. Part Design -> sketch -> draw your pen shape -> revolution to make it round; then sketch -> pocket to make the slot.

+++ for build123d. Just finished TTT tutorial models and it was quite fun. There are some issues with non-regular fillets and 3d offsets but they are minor comparing to FreeCad crashes. Build123d algebra mode is fantastic, especially after your find out how to compose faces from custom line chains. Documentation is good, though many tricks how to get non-trivial tangent points could be found only in examples.

Check out build123d, which has a nicer api for cadquery. I draw sketches in svg if the sketch shapes become too unmanagable to express in code