Bobbin originally designed for classic Elna sewing machines but should be customizable to work for any sewing machine. Our Elna machine came with one metal bobbin that was approximately 21 x 11 mm. This bobbin is by default 20.1 x 10.8 mm although you can adjust it using OpenSCAD or the Customizer (if it's working). I recommend using this slightly smaller size in a 1970s Elna machine as the plastic is not as slippery as metal.
This is a two part design. It is a tight fit and hard to get apart once together. I do recommend applying a dab or two of super glue on the inside of the bigger piece. Then push together the two halves (it may take a little bit of force and you need to align them just right). After the bobbin is together, wipe the inside bore to ensure no glue remains (there is a small empty space that should trap most/all of a small amount of excess).