To cautiously step in here…
@anikolo - Based on what you write in your first post, it sounds like what you are looking for is what is often referred to as a face/face offset mesh.
So both the inside and outside, and all the sides of your cells are planar, the inner and outer mesh have the same number of vertices, and the inside and outside face pairs are all at the same distance from each other.
If just looking at the sides of the cells as beams, this is also an example of a structure with torsion free nodes.
A quad mesh has this property if it is conical - which means all the faces around each vertex are tangent to a common cone.
Closely related are circular meshes, where the vertices of each face lie on a common circle.
Circular meshes can also be offset to another planar mesh, and still ‘node out’ with torsion free nodes, but instead of having constant distance between corresponding face pairs, they have constant distance between node pairs.
You can convert back and forth between circular and quad meshes, and it is sometimes easier to create a circular mesh first and go from that to a conical one.
Also - regarding approximation, when you are modelling anything more complex than a simple orthogonal shape on a computer, there will always be some deviation from geometric rules in the ‘strict Platonic sense’ due to the finite precision used in calculations. We usually don’t worry too much about this, because these deviations will typically be many orders of magnitude less than practical construction tolerances (easily less than the radius of individual atoms).
There are ways of generating face-face offset meshes for certain simple types of geometry through simple short sequences of geometric operations, but for more complex shapes, generally the only way we have is to use some sort of iterative/numerical/optimisation based method.
These methods do change the nature of the design process somewhat, and in cases where a simple non-iterative geometric construction exists it often makes sense to use that, but if used correctly you can use iterative methods to generate far more varied shapes and still meet given geometric constraints to a degree of accuracy that can be considered exact for all practical purposes.