Watertight 3D Model Optimization

I have a set of simple, round printable parts that came from an AI-generated file. Before I can put them on the printer I need you to tidy them up: every surface should be smoothed, all stray vertices removed and, most importantly, the shells must be closed so the slice engine recognises a single, watertight volume.

Those parts also have to work together mechanically. Please propose and implement a practical way for them to open and close—threads, snap-fits, magnets, screws, anything is acceptable because using tools is fine. Explain the approach first so I can sign it off, then integrate it.

Finally, I want to scale the assembly so the “eye” opening lines up with a 1.3-inch OLED (30 mm × 15 mm). The internal cavity of the main body must stay above 80 mm in height after the resize.

Deliverables
• Clean, manifold STL (or OBJ) of each part, ready for slicing
• Assembly version showing the connection system in action
• A brief note or screenshots describing the connection method and the exact scale values you applied

If you need intermediate formats, let me know which software you prefer—Blender, Meshmixer, Fusion 360 or others are all acceptable to me.