When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Permatex RTV (black) sealant/adhesive. Regular silicone sealants will not do the job (for long, anyway). Set the mirror glass on something flat that is a bit smaller than the mirror (face down). Put RTV on the mounting posts and place the mirror down over the mirror. [support the mirror body as necessary] This will keep the mirror from distorting under its own weight {which it will do, if you invert that process}.