Here's a short video about Thomas Hipschen, a retired US banknote engraver. He appears to be using a similar diffuser. I believe that the reason for using this is the same reason as for photography, which is so that the metal reflects the diffuser's white surface.
While pretty much any reasonably sheer, translucent substance should work for a diffuser, you can always hit a photography supplier to buy a purpose made one.
For a reference, click here to see some from B&H Photo Video in NYC.
in the early '90s when I met Thomas Hipschen in his workspace on the 7th floor of the Bureau of Engraving (actually across the street from the press plant where the general public can take tours), it looked like he had a large light table set up leaning over his bench at about a 45° angle. it made for wonderfully even lighting over a broad area. iirc, he was working on U.S. Grant's portrait for the new $50 at the time.
i've kinda replicated that set-up for my bulino work with a 1/16" thick diffuser panel of white HDPE in front of a bank of fluorescent lamps. maybe i should post a pic....
While visiting with the Fracassi family, I noticed that Firmo had placed a sheet of tracing paper as a light diffuser on the window above his work bench. It can be seen in the photo below between Scott Pilkington and I. Also in the picture to my right is Elena Michelli-Lamboy and to Scott's left Firmo and Francesca Fracassi.