I used a bright white-on-white cotton print that I soaked in spray starch. I hung the fabric on the windows and re-sprayed the whole thing smoothing out the wrinkles, etc. It is nearly impossible to take a good picture of-- but I can assure you I no longer have a hankering for the beautiful but pricey window films burning up the intranet. For both the fabric and the starch I'm about 3 dollars out. I'll let you know how it holds up, but so far so good.
*edited to say-- a year and a half later and I'm happy to report that it worked great! I was thinking of switching up the fabric choices, but then decided to leave it alone for a bit. The edges curled slightly on one of the windows (they are up in a room with lots of temperature variance), but I just sprayed it with water the other day to straighten it out.