You really need to start thinking outside the box.

Why not make a squirrel proof extension cord from armoured cable?

Put an electrical connector, electrical box and duplex receptacle at one end of the cable, and fit a male plug as best you can to the other end of the cable. Alternatively, put another electrical box at the other end of the cable and have a piece of wire coming out of that box to a three prong plug.

http://image.made-in-china.com/43f34...d-MC-Cable.jpg

Show me a squirrel that will gnaw through that and I'll show you a clear and present danger to the safety of your country.