This study contributes to the field of additive manufacturing of metamaterials. Design examples are presented to create two very soft microstructures: an auxetic material ($\nu=-1.0$) and a standard material ($\nu=0.2$) that share the same low shear modulus ($G=5.0$ MPa), despite the use of a standard printing material. The design is based on a planar topology, with its unit-cell derived from a RBSM heuristic molecule (HM) composed of shaped atoms with both centered and non-centered spring-based interactions. A homogenization approach is employed to establish a relationship between the stiffness of these spring-bonds and the macroscopic elastic properties of a isotropic Cosserat material. The practical significance of the present HM lies in the fact that: (i) it is modelled through a continuum-based Finite Element model, and (ii) the constitutive description of atoms and bonds is determined based on the elastic properties of the material used in the printing process, combined with the topology. Results demonstrate a good match with the expected properties and a quasi-isotropic response, especially when the deformability of atoms is low.