Tom, I think your clouds look pretty good. To me the most realistic are the hazy ones. The stencil method, since it uses just white paint works best if you keep the clouds hazy. If you look at some of the pix posted, you'll see that sharp-edged clouds also have lots of internal shadows - they are not flat white.
If you do decide to try it one more time, make sure to spray light enough that some of your background blue shows through. Hold the stencil about a foot away from the background, and have more clouds close to the horizon and fewer and fewer as you get higher.
Val