Meow,
There hollow, there the same plastic ones you can buy at the dollar stores that come in the three pack (normally two bones and a hand) I just drilled a hole through them and fed the wire through. The ones leading up to the top skull were a little tougher since I had to fit the plugs through them, like I said all the plugs are plugged in at the top skull, I used a three prong extension cord and actually sealed it into the skull with only the wire sticking out...
