My son is obsessed with Portal. I don't care for first-person games, but the scripting in this one is hilarious. And since GLaDOS isn't a person, I can interpret her however I want.
The mask is made from the shell an old princess-style phone handset. I drilled some holes for the lights to go through; the lens for the yellow light is a coke bottle cap.
The mask base, which goes around my head and is secured with a headband, is made of craft foam. The knobs and dials are made of Legos, more bottle caps, and some large screw spacers. The front piece attaches to it with Velcro.
The dress is a wrap-around that fastens in the back, then belts in the front. In later iterations, I've added a short crinoline to fluff out the shape of the skirt.