if the glass isnt tempered, it shouldnt be too hard. you can just get a glass cutter, score where you want to cut the hole to be, and crack it if it is tempered, you have to drill through it. keep the glass well lubricated and cool
Looks like a 2.5 to me. Even if you notch the frame the top fits tight & won't give you enough room for the cord. Cutting a notch with a glass cutter is impossible, you can't cut right angle cuts that work inward, I used to do stained glass. You can cut a corner off at an angle, or bring it to a glass shop & have them grind a notch for the cord.