Quickest and easiest would be a taxi cab from the YC or BC Bell Desk. They can call one for you no problem - that would certainly be easiest.
Next best, depending on when Magic Kingdom closes, would be to take the bus to Magic Kingdom and then hop the monorail over to the Poly).
However, if MGM is open late that night, you also have the option of a stroll and/or ferry ride to MGM and catch the bus to the Poly.
Hope this helps.