This hanging mirror is actually a medicine cabinet door.
Rough frame in the opening with 2x4s, allowing an extra nailer for more flexibility in placement of the hinges.
Trim out the inside and add shelves.
Attach a lighweight mirror with hinges glued or screwed to the mirror and screwed into the nailer. Glue a set of strong magnets to the mirror and the inner trim so the door will close.
