To get an orthogonal fcc(111) supercell:
1. start out from the primitive fcc cell, e.g. silver.
2. Builders --> Surface (Cleave). Choose the [111] Miller indices.
3. Bulk Tools --> Supercell. Choose Base-centered Orthogonal, and click Transform.
The resulting supercell is orthogonal, but the ABC vectors are not aligned with the XYZ vectors. This is easy to fix: Use again the "Surface (Cleave)" tool, but select the [001] Miller indices in order to retain the fcc(111) facet.