I just completed my own version of a cheap-assed wind blocker. It's not quite as sophisticated as Briano's design, but it works pretty well. Here are some pics:

The fabric is 100% polyester chiffon, from the local Jo-Ann Fabrics. It was on sale for $5 per yard, then marked down an additional 50%. I used a little over a yard. (I actually bought four yards, because I wanted extra in case I screwed up.

) Other materials used:
- (1) 3/8" diameter, 36" dowel
(1) 1/4" diameter, 48" dowel
(1) 42-60" adjustable shower curtain rod
(3) 10" mini-bungees
Black spray paint (for the dowels)
Thread, etc.
My sewing skills had atrophied considerably since I was in 4-H, but i got the thing together. One nice thing about polyester is you don't have to hem the edges to keep them from fraying; you can just singe them with a lighter and the threads will melt together. I did a rough drawing before I started; I'm
including it here in case someone wants measurements to build something similar. If you're short you might want to subtract a couple of inches from the length -- I built in the assumption that the seatbacks would press against the fabric and stretch it a bit.
So how does it work? Pretty darn well. My ponytail no longer comes around and hits me in the face. The dominant wind direction is now upwards, not forwards. The car is now comfortable on the freeway with temperatures in the mid-50s, whereas before I would have wanted it to be at least 65F out before driving at speed.
A disadvantage compared to Briano's design is you can't access the back seat from the front seats with the wind blocker in place. Also, the fabric creates some wind noise, although it's not nearly as loud as the battering about the ears you get from the turbulence without it. The view out the back is dimmed and blurred a little, but you can still see what's back there.