I'm using this bare speedo wire core for the animatronic tail/spine tentacle. It is excellent for this purpose and is relatively inexpensive. There is no need to buy a complete speedometer cable assembly from your local auto parts store and take it apart. that's exactly what you want. (See my review of Wire Rope Basics - Tentacles.) Use a good wire cutter to cut it to length; I suggest leaving something that you can trim later. The attached photo shows the 3D printed "spines" with a string for tensioning and a Dorman 03368 speedo cable for the "spine". This works great for this purpose.
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