Consider this: Take a piece of glass (ground glass, anti-Newton glass, or glass treated with the Tetenal spray) and on the non-reflecting side use black cloth gaffer's tape to make a film channel, and light calibration notch. Place the film on the scanner emulsion-side down, and then lower the glass and orient the film under it. While the film won't be clipped into the "holder" it will be held flat and in a reliable position.
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