Late to the party here, I know, but in case it helps, you can get glass from Focal Point which fits that holder. They sell ANR glass and a thin "normal" glass. Yes, you remove the clamp (I seem to recall there's an instruction paper with the glass).
That said, I have this myself but have found that even with ANR glass my Coolscan 9000 occasionally gives Newton rings. It is supremely annoying and because it is not possible to predict when or which type of images will have such rings, I have given up on using this tray with the ANR glass.
For a while I used only the thin normal glass which helps flattening any curled negs. I have seen no degradation at all by inserting that extra glass between the film and the CCD.
Ultimately I gave up on this and looked for an old Image Mechanics wet mount tray, which eventually popped up on eBay. That gives truly flat images and slightly higher scan quality too. Unfortunately they're not easy to find. I bet such trays could be 3D-printed today, though. It's not at all a hassle to wet mount. But I digress, sorry.
Br
Philip
Chers Philip. I assume you mean a mod on the FH869S and removed the clamps? Where did you get the glass?