using a cheap mirror from Walmart or Target: Attach to a piece of plywood (paint it first), then add either crown molding or any other type of wooden accents. Although I've never seen cheap mirrors that wide at walmart.
DIY Large Mirror Frame using a mirrored closet door. Incorporate a piece of paintable textured wallcovering to the inset panel above the mirror. Add decorative trim or molding for an extra design motif.