  We present a regular cubic lattice solution to Einstein field equations that
is exact at second order in a small parameter. We show that this solution is
kinematically equivalent to the Friedmann-Lema\^itre-Robertson-Walker (FLRW)
solution with the same averaged energy density. This allows us to discuss the
fitting problem in that framework: are observables along the past lightcone of
observers equivalent to those in the analogue FLRW model obtained by smoothing
spatially the distribution of matter? We find a criterion on the compacity of
the objects that must be satisfied in order for the answer to this question to
be positive and given by perturbative arguments. If this criterion is not met,
the answer to this question must be addressed fully non perturbatively along
the past lightcone, even though the spacetime geometry can be described
perturbatively.
