If you're like me and not got steady hands, this will make centering the mirror without the use of guides which require steady hands a breeze. And all it takes is about a minute.

The inner printed side facing the mirror (no need for outer circumference) needs sellotape pressed (stops scratching mirror surface) and trimmed with a very sharp implement like a scalpel, these can be purchased cheap. The outer perimeter will fit snug around the glass, so if there's a beveled edge, it will still find center.

Reinforcement labels I used were Avery 6755.

PLA+ was used with a .3 nozzle , layer height of .16. Takes about 30 minutes at 40mm/s

.4 nozzle might work but have not printed