Patrick Chase
Yeah, 3M lapping film would work.

That's rated in microns, where 120 um is about 100#. The 60 or 30 um stuff should work nicely. I have some 3x8 sheets of the 30 um that I've used for stone flattening, though it ends up being a pretty expensive option.

I'd use float glass rather than a granite surface plate. The surface plates have a bit of texture, and the diamond films are thin enough (3 mil polyester) that the texture can telegraph and cause "hot spots" on the film where the abrasive wears prematurely. Glass has a much more uniform surface.