This looks like an old question (2016?!) but it's on the front page and still open. You don't need that extra geometry to apply a bevel effect via normal map. Just bake the normals of a high-res beveled cube to a simple cube in Blender (free software). There are many tutorials for this on Youtube.
I don't think it will look incredibly realistic but it will not look worse than what you have with more geometry, and require only 6 faces.
