2 PET bottles; 1L and 500mL bottle
With your ruler, measure 1.5 inches from the top and bottom of the 1L bottle. Cut both parts off.
Measure 5 inches from the top of the 500mL bottle and put as point on it. Measure another 6 inches from the top of the 500mL bottle and put a point on it again. Connect the 2 points together to form a diagonal line on half of the bottle. Cut through that line using a pair of scissors or a cutter.
Connect the bottom part of the 500mL bottle to the face of the 1L bottle with electrical tape.
Final Image=Megaphone!! :)