actually, there is a simple solution, where all surfaces are G2. The video rhinouserx linked above shows the right method, it is just executed in a sloppy way. When you build the fillets from degree 5 curves and select curvature continuity on two sides of the fill-in squares, all you have to do is to pick up symmetrically adjacent control points and move them together slightly with the UVN tool (use the normal direction). That a great result in the end, and it’s pure Rhino.
The benefit is, this method also works when the underlying geometry is not a cube, meaning when the sides are not perpendicular, or when the sat back fillets have more crown/bias (“pointier” fillets).
Maybe you want to give it a try…
Typical solution.3dm (735.8 KB)