Thats the one thing I wish the givia would expand out enough to put a helmet in.
Sometimes the rear bags help with keeping the wind up and over stuff.
when I trailer my bike, the MPG towing the bike is not that bad, it's actually about w/o the trailer, I find the bike kind of splits the airflow and sends it to the sides like a wake on a boat. WHen the bike is off the trailer, it's horrible, the back gate is like a huge sail now grabbing the wind rolling off the top of the truck. The bag would kind of do the same thing, keep the airflow from churning and becoming turbulent until AFTER it's interacting with the bike.
Aaron