I did it with JB weld, after getting tired of Permatex mirror adhesive failures. I scotchbrited the glass and the mirror mount to give the epoxy a rough surface to grip. I then used a big gob of JB Weld epoxy. Squeezing out around each side. I taped it into place, using my finger to carefully mush the epoxy back on to the mirror mount.
After it cured, I had to dremel away the epoxy to get the mirror to slide back on.
I may have let it cure for a week or so, before putting the mirror back on.
No failures after that. But, I did have a bit larger glue surface area to work with, due to the way I applied the epoxy.