I used a pneumatic die grinder with a cutoff wheel. It cut like the proverbial hot knife through butter, and was easy to control. I was able to get so close to the finished size that it only took a few strokes with a hand file and some wet-or-dry paper to clean the edge up nicely. Like the others, I covered the area inboard of the Sharpie line with masking tape so I wouldn't scratch the lens. I did all the work in my hangar where the official temp was about 45 degrees- no cracking- though I wouldn't take that chance with the canopy.