Not sure how thick the glass is, but I would guess with the very first attempt to remove material you will know if the process you have chosen is going to work. Chipping will be the devil that needs to be controlled. I would make my strokes or work from the front side painted and into the opening. When done sand the upper and inner edges of the opening so they are rounded and feel smooth.

An aggressive file might work the best, saws, power air tools can get away from you real quick want to remember that if used, light pressure to start. Air offers the most control because you can roll back the pressure and slow the tool down, think die grinder.

I'm sure you will mask off the area and draw two lines the outer edge of the trim ring and the actual opening for the bucket. Just stay with close tolerances to the opening and you will have things come out nicely for you.

BTW when I say aggressive file I am talking about something like this, it is a half round file.

Example:

Nicholson Wood Rasp File, American Pattern, Wood Rasp Cut, Half-Round, 10" Length: Amazon.com: Industrial & Scientific

Have fun modes are always cool as hell when done, wow factor for the modifier is 10.

Good luck,
Greg