Hi Pilou, it could be possible that a Parasolid based modeler like SolidWorks or NX might do a better job.

But even on a box the shape of the patch is a little different depending on the particular radius and setback values used, it's not like there is only one single corner shape that fits every kind of case...

The type of solution that you're talking about with just reusing one specific surface would only work on one shape and one set of parameter values.

- Michael