many people make often a big deal of something very trivial.
I just made a 8x10 ground glass and 56mmx56mm for a Meopta Flexaret. Use the glass from the cheapest photo frames.
Use silicone carbide in 400 (for a first easier sanding) then in 600. Could use finer, but 600 is quite good.
The tool: a sanding pad glued to a piece of glass:
the point is not the sanding, but the cutting of glas.
Here the focusing screen just made for the Flexaret. I destroyed two 10x15cm photo frames in order to get an acceptable result, the upper left corner isn't straight, but it doesn't matter for using the camera. And brightness is good:
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