For those worried about being “seen” I don’t think it’s a problem. The “natural” state of the plastic/glass material is probably opaque. When voltage is applied, the composition changes and it becomes translucent. When a person is inside and the door is locked, the voltage is off, so the glass is opaque, and the user is not visible. Upon unlocking the door and leaving, voltage is applied, and the interior becomes visible so the next person can see that it is vacant and available for use. Simple.
The optimum material is polycarbonate: a specific gravity 40% of glass, indestructable, naturally translucent and dirt cheap; but can it be formulated to change opacity with electric current or other input. This requires American ingenuity.