Here's a quantity of FN11 glass lens blanks. They're 102.1 mm diameter, 27.7 mm thick at the edge, and have been cast with ~5 diopter negative curves on both sides. FN11 glass has a refractive index of 1.62, and an Abbe V number of 36.2. We have a decent quantity of them, so you could plan a club or classroom activity around them. Weighs over a pound. Unused.