To shoot on a day with light wind, you may want to build yourself a windbox, made out of Plexiglas. Quite simple and you can place your diffuser on the top.
Check out this link and build your own windbox. http://www.youtube.com/watch?v=HUhbhXbslW4 Well known macro photographer Mike Moats shows you how to build it. He has three different sizes, thirteen inch, eighteen inch, and thirty inches (with a width of 15 inches). I built the eighteen inch size. The largest, 30 inches, is for the tallest wildflowers.
I bought some Plexiglas at Rona, and they cut it for me. (One cut didn't work out very well, so I covered the edge with extra duct tape.) Mike says that if you buy it at Loews they will score it and snap it for you. (That was in the States though) He also uses something called Gorilla tape for the edges. Couldn't find any up here, so duct tape had to do.