Im looking for a good way to get diffused light outdoors that will be relatively windproof. Umbrellas act too much like sails (especially larger ones). Now in video I usually would just use a second lightstand with a big, flat, lightdisck-like white cloth. Shooting the flash through this diffuses it pretty well.
Would a similar thing be good for photo? It might catch wind less but it is inconvenient to have two stands.....
If you are talking about diffusing the light from the sun/sky, you be in trouble period. They all catch wind. Most reliable is a heft, strong assistant.
If talking about diffusing light from a flash, you can get transparent matte acetate sheets from a good art supply store and cut them out to fit the flash head.