fillbelow.lua

slot=16
item=0

-- move over first 'hole' column
turtle.forward()

while not turtle.detectDown() do
  -- filler starting at end
  turtle.select(slot)

  y=0
  while turtle.down() do
    y=y+1
  end
  for z=1,y do
    turtle.up()
    turtle.placeDown()
    if item==65 and slot>1 then
      item=0
      slot=slot-1
    end
  end
  turtle.forward()
end