The Parabolic Softbox AD-S85S for AD300Pro and AD400Pro measures 33.5" and allows you to choose between a silver or gold softbox (umbrella) by including both. If you want a cool-toned look with snappy contrast, attach the silver reflector inside the umbrella. If you want a soft, warm rendition attach the gold reflector. The removable front diffuser makes it easy.
The AD-S85S shape has a deep design to output the soft light you're seeking but with a sharp falloff to make feathering and edge-lighting simple. The fabric grid thwarts spill light and gives further directional control necessary for selective lighting.