I routinely weigh down the scanner lid with an extra hard foam backer to flatten out warped archival material tight on the scanner glass. In the old days we had vacuum frames to flatten things out...perfectly! I've had no trouble with using 16 - 18 pounds on the glass for years. Today I tried 22 pounds on an Epson V600 and it started to break. When I get a chance, I will write up a proper report, but too busy right now scanning! Be careful how much weight you put on your flatbed scanner...they will break!
I learned this lesson a LONG time ago when some NITWIT at work tried to take a picture of his ASS on the copy machine. He was replaced the next day by someone a little smarter.
We're talking about a thin sheet of glass, after all.