Calippo, toothpaste and milk .. packing

Solution 1:

The convex hull of a circle and a line segment is developable. I don't know if this is the unique developable surface with the given boundaries.

enter image description here

(Ignore the extra chords drawn in the circle.)

I'm fairly sure that if you start with a cylinder and pinch it flat at both ends, a perfectly flat tetrahedron is the only developable surface you can get. The curved nature of the real-world milk cartons is because the material is slightly extensible (it has a coating of plastic to make it liquid-proof). Better intuition can be gained by trying the experiment with a sheet of plain paper instead.