While I have not tried a plastic bag, a common cotton kitchen towel will work as well. It should probably be thicker than the floursack towels commonly used to dry dishes. It should also be thick enough to handle some stress ,
Roll it up a bit with one corner pointing out.
Turn the carboy upside down.
Stick the towel into the carboy so the towl forms a loose funnel inside the carboy neck.
Shake the carboy (no easy task) until the stopper rolls into the towel funnel.
Pull on the towel so the stopper comes in towards the carboy neck.
Put the carboy on the floor (if done right, the stopper will stay in the towel).
Pull firmly and the stopper will remain in the towel as it is pulled out of the carboy.
Repeat as necessary.
Post responses to forum post when others perform experiment.