Hehe... that does an excellent demonstration of the turbulance caused by a gigantic tube floating in front of your car as you drive down the highway.
The oh-so-reliable myth busters uses a whole box of straws. They cut off the top and bottom of the box and they instantly have a packed square of staws. You may even be able to find a packed square the same size as your tunnel.
do you have the fan sucking out or blowing in? Just occurred to me that sucking out may give you smoother entry air than a fan blowing inwards.
Good job
Just need a really good model of your own car now.