The .358 Winchester can be easily made from .308
Winchester cases. :-)
Simply make, or have someone made for you a tapered case mouth expander plug to open up the .308 case mouth to .358 dimensions. You can modify the existing expander plug in your .358 reloading die set using a lathe, and hand polish using 400/600 abrasive cloth.
The cases are modified in one stroke on a reloading press. Dip case mouths in powdered graphite and run them up into the expander plug.
HTH
John