Tube Along Path will accomplish this also. There is also Pipe Along Path.
Lines to Tubes and the two I mentioned have slightly different results under certain circumstances.

You could also use Tgi3D SU Amorph's "create mesh" command. There is a free Training Edition with a limit on the number of vertices to work with. It may be sufficient for your model here.

Simply select any closed loop and click "create mesh". Create mesh generates mesh surface inside a given boundary with optimum smoothness.