I've been experimenting with the penny technique, and it works unbelievably well on many kinds of nibs.
American pennies are ubiquitous, in the United States anyway, and the last penny with the Lincoln Memorial on the back was minted in 2008. Current pennies display a union shield on the back. So you're limited to the millions of pennies minted from 1959 to 1981, if you want real copper. Or from 1982 to 2008, if you just want the ladder/file-like Lincoln Memorial.
This is a borderline obsessive-compulsive focus on arcane detail, I realize, but since you're reading a thread about little-known nib polishing tools, you may be familiar with the disorder.