We now want to fillet the edges at the intersection between the Snap Fit and the battery cover solid. A Rule Fillet can do the job.
We do not want the fillet around the top edges (red arrows). The edges are included because they share the curved face of the Snap Fit that coincides with the curved face of the battery body. They are called "merged faces." We can skip such merged faces (and all the edges they share) by doing the following: