If it's an original New Model Russian - use the original coin.
You 'can' use sheet sterling cut to size, though - almost any decent jeweler can help.
I use original coinage - down to and including the 5-Peso conchos on my spur leathers, and the Mexican Revolutionary monies for a later Impression.
I had had a bunch of Third Reich stuff that absolutely fascinated a local coin collector, so a trade was in order, leaving me with a leather reel bag of period coinage in 'nice' condition - exactly the kind of condition expected at that time.
Vaya,
Scouts Out!