Check on steel electrical conduit at the hardware store. There might be a size that would serve effectively. The reality is that a steel liner will not ad much strength o rigidity to the structure because it's closer to the center (neutral axis) and the modulus of elasticity is too different for the steel liner and carbon tube to function as a single structure.
However if it extends well below the headset, a steel liner may act as a safety net staying intact if the carbon cracks.
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