I've put an IKEA "LOTS" mirror on my build plate. Clean it with vinegar to get good adhesion.
I used to keep it in place with binder clips, but didn't like the fact that those clips would sometimes bump against my BL-touch sensor. I've mounted these with cello-tape.
First mount front-left corner, then back right, then the other corners. Seems to work OK.
Maybe it could be better tuned to the exact dimensions but this works well.
Feel free to tune it better.