a) Drill 3 small holes on the sheet, as near to the edge as possible, as far away from one another as possible.
b) Hang a plumb line from one of the holes.
c) When the sheet is stationary, trace the vertical line made by the plumb line using a marker.
d) Hang the plumb line from each of the other two holes and repeat the tracing of the lines.
e) The intersection point of the 3 traced lines is the centre of gravity of the plastic sheet.