I use glass plates with my Betterscanning holder and Epson V700. Bought from a framing store. This is so-called "anti-reflection" glass, actually slightly frosted; not vacuum coated museum glass (much more expensive). I use it just on the top side, with emulsion down (activate mirror option), to flatten the natural film curl. I have not seen any sign of the frosting pattern showing up in scans. I saw another forum post sharing the same experience.
This "anti-reflection" glass exists (at least around here) in two varieties, with one or two frosted sides. Obviously, if you have such a piece of glass between the film and the scanner lens, only the side against the film should have the "AR" frosting. Ideally, all surfaces between film and scanner lens should be "true-AR" (vacuum coated), like in the Epson V750/V850. An idea for upgrading a V700/V800. But I'm digressing.
Given the price (and availability??) of allegedly "photographic" ANR glass, you don't have much to lose following my suggestion and making a trip to a framing store.