How about moving the medium above the Flow super (switch positions). That way they will have to walk across the plastic to get to the good-smelling stuff, and they will probably decide to use the Flow frames.
Edit: Oops, I misread what you wrote. OK, so if the medium super is fully capped, I would remove it. If not, leave it above the Flow super. If you haven't already, I would also suggest getting some wax onto the Flow frames. You can do that by smearing some burr comb on with a hive tool, rubbing a bar of wax over the frame faces, or painting on melted wax. That might make a big difference.