You don't  need a straw, or a tube, just close your eyes and shoulder the shotgun, then open your eyes without moving any other body parts. Are you looking along the rib, or down at it? Is the rib centered left to right?
  The biggest mistake  that many new shooters make, is closing one eye, and trying to aim a shotgun using the beads as sights.  With a shotgun, you keep both eyes open, and concentrate  on rhe target, not on the beads.  Trying to aim a shotgun  like a rifle usually results in following the target way too long, and stopping your swing. You need to keep the gun swinging, until after you pull the trigger, or any lead you have established  is lost, and you shoot behind the target.