The AN glass that comes with the Betterscannig frame might be ideal. Their glass for 120 film is an exact fit for the 120 holders of the Nikon 8000 scanner and presumably the 9000 too. Something to try when I've finished repairing the bathroom door.
David, I use their AN glass on the Epson fluid mount, hinged with tape as in the video, and on the Nikon 9000 120 holder. Works extremely well on the Epson holder, however, on the Nikon 120 holder it keeps the negs flat but I found I got what I can only describe as edge burn between the frames spaces, I managed to pick up a Nikon glass holder with film masks that cured that.
I'm not adverse to placing the film directly onto the 'glass' (ie no film holder when scanning my 'occasional' 8x10 negaatives. I had to do it that way when I first started scanning my 4x5 negatives when a 4x5 film holder that came with the scanner was a 'tad' too tight and the film would not 'fit in properly. I contacted Epson about a 'how to fix the problem. A few days later later I received a 'replacement' by 'Special delivery' and an 'instructions' to seturn the 'bad' holder ASAP.. and to send "collect.
That is but one of my reasons for recommending the Epson.
And then... just the other day I had an invitation to 'hook up my scanner' for their newest software "update". When I have the time
(after spending some time sclearing the latest deposition of 'extremely cold white 'stuff' fell from 'up there.... about 8 inches ( I 'think its more of the 'well into the "minus degree temperatures freezing the atmospheric humidity' rather than from 'snow from the clouds'.
These 'snow-'flakes are only 'about 2 to 3 millimeters 'across.... but just as 'cold'