I invested in a piece of Tru-Vue "Museum Glass," and although it is rather pricey, it works very well. I do however, pay great attention to the surface of the non-reflective film, located on the back of the Museum Glass, since I do not want to scratch this surface. Lastly, you should not cover the calibration gap with your makeshift piece of glass, since this will introduce banding in the scanned image.

jim k