I just got one of the 120 ANR glass pieces mentioned below for 120 and gave it a quick test today. I did not have the time I needed to get really detailed but here is what I found so far.
I have the 35mm glass from Better Scanning and they are fine. They seem to be ever so slightly thicker than the new 120. BS seems to have a more rounded edge not that the new one is sharp or poses a problem. The glass is big enough and heavy enough to keep the 120 films flat. I scanned some film with the Epson software at 3200 (Zeiss Nettar 6x6 – 52MB ea) and one with Vuescan, (Fuji GF 670 – 6x7 is big) using Chris Crawfords notes at 6400. (Thanks, Chris.)
I took them into Photoshop and adjusted as needed. My conclusion is this…. They serve the purpose very well. Flat film and a really good scan. Definitely better than the Epson film holder alone. I blew up the negs in PS and I was really surprised at the detail. Recommended. A good solution without having to buy a new film holder.