A real stethoscope has some features that make it useful in normal use, but would be a hindrance in your scenario.
The sound conductance medium is the air column in the tube, not the tube itself. Between ships or spacesuits, the air column would not be present. The flexible material of the tube is a poor sound conductor.
The ear pieces help to direct the pressure waves to an eardrum. They will couple poorly to anything flat like a ship.
Based on no evidence, I'd assume a rubber/plastic stethoscope would be almost useless. They're also pretty short. If you're close enough to use one, you'd probably be close enough to be in direct contact which should help.
If I needed to construct some sort of "tin-can" system, then I'd want it to be rigid to reduce losses, and to couple better with rigid exteriors (like a panel or faceplate). It would be nice if it had some flexibility so that you don't risk poking a hole in things. My first thought is modifying something like a cheap curtain rod or a telescoping radio antenna. It has rigid tubes that will transmit sound. The tubes can pass by each other so that the length isn't fixed. Adding a cone or disc shape to the ends to increase the contact area would probably help.
The utility from such a device seems very low. I can't imagine too many situations where every it would be useful and less risky than other means of communicating.
Whether it could be used on the skin of a spacecraft depends on the makeup of the craft. The more insulation and separation between the skin and the inner walls, the greater difficulty this will have.