My conclusion is, that while the RB-120 is a great deal and a fantastic light source when used as intended, I don't see any reason to adapt it for indirect mounting or focusing. It's simply not parabolic enough to have a focal point and function like a Broncolor para 133. I'm going to revisit my idea of making a fully DIY deep parabolic.
No, but we figured as much, based on your informal examination of the shape of the Ricebowl, as mentioned in your earlier post.
The one thing I would like to try with the RB-120--since I find that the Broncolors are the most magical when defocused to the point where they are a large ring light source--is replacing the inner white diffusion screen with a round silver reflector (homemade or a popup disc type), ideally black on the non-silver side, snapped in place of the diffusor with the silver side facing the strobe at the rear. If I can get all the light coming from the outer rim of the softbox, perhaps it will have a similar look to the Bron?
I'm curious. Why would you build your own when there are some good tools readily available? Is it for curiosity's sake? The bold adventure of it all? The mountain you need to climb?