So I was thinking of one of these.
Put on a wig under your helmet if you need more space. I can't remember where they did the study, but one of the findings was that drivers gave more room to riders they thought were female. Another way to get more space was to not wear a helmet. I guess the (drivers') assumption was, if they have a helmet, they'll be okay. I wish I remembered where I saw that study, so I could post a link.
A guy actually showed up to a bike event with something similar to that on his bike. He had a fiberglass pole attached to a rack on the back of his bike. He duct-taped a red blinkie to the end of it. It extended at least two feet out to the left of his bike. While I expect that it is effective, I don't think it is neccessary.
What I found dumb was that he was riding with it at an event with 50 or so other riders. The thing was perfectly aimed at everyone's spokes as we rode by him. We also had to ride out in the road to get around him. Not very considerate of his fellow riders. Thankfully, I only rode near him for a few minutes.