I used to cut out the convex mirror to just the right size. It was difficult because glass was put on it unless it was scraped carefully, but it worked well. The price is also cheap and it seems to last a long time as much as cutting thin glass. I bought rough things of the same number at the same time but it was not suitable for sharpening thin glass.
I purchased a transparent glass plate (t3) to shave it about 1 mm. (Shaved length is about 80 mm) (It is difficult to cut 1 mm with a glass cutter.) Although there was no major collapse, it could not be cleaned beautifully. It seems that the side part is wavy, the cutting trace is biased to a part, probably because of that. If you want to cut the glass cleanly, it seems that a grinding wheel dedicated to glass is good.