Back of the Canberra. Take out long Perspex nav rule and put one end on the nav table directly in front of you where it met the instrument panel. When safely wedged in place lean forward and put the other end of the rule up against the brow of your helmet and then continue to move forward gently until the rule becomes jammed between the helmet and it's visor. Thus braced one could doze happily for an hour or two.
Particularly useful on West Drayton PIs.