How to set thickness to a box surface

After creating 5 faces (open surface) of a box, I want to set thickness to the wall of face to becomeclose polysurfaces.
How can I do?
Thankssurface.3dm (42.0 KB)

It’s not necessarily a good idea to make the walls of this object as one single surface with creases. I would make these as 4 separate surfaces.

That said, you can join the two surfaces that you have and either use OffsetSrf to get a closed object with thickness or cap the surface and use Shell to offset to the inside. It seems that the latter needs some cleaning up as well…