A set dekorator I worked with used Chalk in water, to diffuse windows. Very cheap and you can easily clean it afterwords.
Something else I've used when I was on a budget and needed huge diffusion was the foliage used on construction sites over here.
You get them in different denseties, sizes and even Colours and they are water resistent but let wind through.
Great bang for the bug and easy to rig because of the many eyelets!