Actually, I think you would want more of an hourglass type shape. Sort of a double teardrop. You would want the leading edge of the port to be very wide so it will flow well even though most of the port is still closed. Then get more narrow so the port doesn't get huge and kill your velocity. Once the leading edge starts to close you would then want the trailing edge to start getting wider to make up for the area being lost at the leading edge.