I've had good results using 600 grit emory on a random orbit sander, on 2mm Lucite sheet plastic, available from big box hardware stores in US. It's not glass, but it's also inexpensive and easy. And the results are pretty good.
I've also done the same with the 8.5"x11" plastic fresnel sheet magnifiers, available from Staple's office supply. The reverse side is flat, and takes the emory sanding pretty good. Then orient the sanded side toward the lens, and you have a GG with built-in fresnel.