Measure the width and height of your mirror. Calculate how much edge you need by subtracting 400mm (the corner pieces) from each dimension. Divide this into pieces that will fit on your print bed. You can print diagonally if you need a little more length.
Print 4 corners and the edge pieces calculated in the previous step. Stretch the edge pieces in your slicer as necessary to get the correct lengths.
Put two-sided tape on the back of each piece. Use clear tape, since it will be slightly visible in the mirror. For a more permanent install, I used "mounting tape" which is clear once the red backing is peeled off. Glue might work too, but may not look as clean.
Don't use long strips of mounting tape like I did. Small pieces provide plenty of strength.
Tape the frame to your mirror. The last piece you attach should be a top corner. That makes it easier to hide any slight misalignment.