Cut a hole in the cooler large enough for the heating pad's electric cord and plug to pass through. Cut another hole above the first hole, making it the size of the heating pad's control panel.
Line the bottom of the cooler with aluminum foil. Place the heating pad in the bottom of the cooler and tape the edges of the aluminum foil to the sides of the cooler.
Pull the heating pad's cord through the lower hole in the cooler and insert the control panel into the other hole. Cover the inside of both holes with aluminum foil.
Place a towel on the heating pad and close the cooler's lid. Plug the heating pad's cord in and select your desired setting on the control panel.