Plane Stress - 3D Problem

I want to simulate a 3D case. However, for simplicity, I approximated the problem to a plane stress case.
I used this example,, to implement the 2D case. I considered only a 2D case, the z components are all null. However, in a state of stress, the strain zz is not null. In this way, the results obtained are different from those obtained in commercial software.

I think the difference lies in not having given the thickness of the mesh element.

How do you implement a plane 2D stress case and provide the mesh thickness?
Is it possible to solve this kind of problems in Fenics?

@bleyerj’s tutorial here shows how to convert a plane strain code to plane stress through a modification of the material parameters.