What material to use for a capacitive sensor?
There's an Adafruit capacitive touch beginner tutorial here that explains that the larger the surface, the more sensitive your touch pad.  If you take a piece blank copper PC board and solder a wire to it and then connect that wire to your capacitive touch sensor, it should be able to sense through the enclosure.  The key here is that the enclosure must be non-conductive plastic.  You could also use copper foil tape, but that is probably more expensive than a small piece of PC board.  Just make sure that whatever you use is attached to the surface with minimal spacing.
If you made an actual PCB, you could put the sensor in the foil on the back of the board and stick that against the inside of the enclosure.