If it is uniform thickness, you can import the inner surface(as surface body) into Mechanical. Create a surface mesh. Then use Pull extrude to create the solid mesh from the surface mesh
Surface body is suppressed after creating the extruded solid mesh
Example:
Consider setting quality check to No in details of meshing if the aspect ratio is high. SInce the geometry has high aspect ratio, the resulting mesh might have that as well.
Or consider using a shell mesh and define thickness as a property
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