dry cardboard should have no trouble standing up to the "pressure" of normal house current. WET cardboard, not so much.... but then again WET anything is not usually considered a great insulator ;-) the only purpose the cardboard serves it to prevent inadvertent contact with the wires in the plug and the cover screw on the outlet, once the thing is plugged in it is not like you are going to get your finger in there anyway. I guess it also keeps stray straight pins or nickles out when it is not plugged in... that would result in a "pop" and a startled expression for sure.
I guess what I am saying is about anything NON conductive to keep junk out will be fine.
Last edited by greywuuf; 05-27-2015 at 08:31 AM.