This is the second in the Galaxy mirror attempts. I'm getting better with the air brush but sure do need a lot of practice. I also used a dovetail bit in the router instead of the 1/2" straight bit. I like the sharper angle for this type of cut in the mirror.
I just use a 1" square block and CA glue it to the inside of the mirror. Sometimes as in this case I had to round over one side so it was more like 1" x 3/4". I hold it with my chuck. Sometimes I will use a rectangular piece of wood and remove 2 jaws from the chuck so I can more easily position the piece where I want it.