I've never done something like that but maybe some of my ideas are doable.
Hmmmm maybe brass etching? One single piece and then bent them around the head.
Or use a really fine wire. Glue it on in the shape of the sunglasses and glue plasticard or thicker metal foil onto it (Not the thin kitchen/backing variaty.) I have some stronger 0,15
mm metal foil. But it would be really hard to cut into the small lenses.
Directly sculpt them on and try to hide the fact that there is no space behind the lenses with some fancy painting.
Use something like this:
http://victoriaminiatures.highwire.com/product/goggles-hex-x5
Or something similar(if exist) and try to trim the belts to look more like the sunglasses of choise.