The tube is considerably larger than the vehicle, so really it's only the smoothness of the bottom part of the tube that is important, not the precise dimension

The tube is considerably larger than the vehicle, so really it's only the smoothness of the bottom part of the tube that is important, not the precise dimensions. And the surface the vehicle flies over doesn't need to be steel - you could line the bottom half of the tube with a wax, or something similar, and that would be much easier to smooth to the tolerances needed, and re-smooth if it suffers any damage. In fact you might be able to handle some thermal expansion (at least seasonal, not daily) that way - release some clamps on a telescopic section, adjust, reclamp, melt and re-smooth the wax.