How do I give thickness to an abstract lofted surface?

 I have create a lofted tunnel surface but now I am unable to give thickness to the surface / extrude the surface .

I have attached a simple file to give an example of the type of form I am looking at giving thickness to. I tried offset surface but that does not give the internal thickness.

What would be the right definition to extrude / give thickness to such surfaces ?

Thanks

Ash