This is what i did,In the first image I makde a layer and set it to 1 yard altitude, and 2.5 yards height,quite simple and more of a swamp type mist or fog found in nature,in the second image the same material is added to a cube, disadvantages are render times. The standard cloud layer takes longger to render but looks nicer than the cube and depends greatly on the size of your scene. You can of course add basic functions to getr a swirlie effect or more of a clumpy effect no problem.

"Many are willing to suffer for their art. Few are willing to learn to draw."