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Not trying to discourage your DIY efforts here, just sharing knowledge gained from already traveling down that path and using speedlights and studio lights with various modifcation techniques.
Wow, that's an impressive piece of reply there. I recognise the images you supplied, I've browsed through your web site at some point in the past. And for a mobile studio setup, I will certainly think about the umbrella route. I suppose you're talking about shooting a mirror umbrella, bouncing all light back, and not a diffusing umbrella?
About the DIY softbox: it's all finished, though it's using the wrong fabric for the front diffuser. I am very happy with how the light works on people. It's a very bulky and heavy softbox that is meant for my home studio, so I won't be needing to move it around. When I use the softbox only, and correct the colour in RAW post processing, I can get very good results using 400-800 ISO.
I used aluminium foil in the interior of the sofbox. The SB measures 60x90 cm (24x36inch) at front and it's 55cm (22 inch) deep. I created an inside diffusor by wrapping some aluminium foil around a small tennis ball sized half dome shape and positioned it at about 4-6 inches from the speedlite. It bounces the light to all sides, so I don't get any direct flash beams emitting through the fabric.
I previously didn't have the inside diffusor, but another sheet of the thick fabric placed just 2 inches in front of the 580exii flash. That took another 2,5 stops of light, so I could only photograph people standing at 4 feet from the SB. Now I get much better results and hopefully, with new fabric, I can use the SB from 10-15 feet away.
I'm using this SB because it cost me about 30 Euro's to build the softbox and I want to experiment with studio setup before I go and spend $2000 on "real" equipment. I think gaining first hand knowledge about positioning light sources is much more important than having top gear. I can set my ISO to 1600 or 3200 if I have to, just in order to learn the basics.
As soon as I know what I'm doing, I will probably get an Elinchrom 500Ws flash set (two flash heads, two softboxes, 1 umbrella, 2 stands) and combine them with the speedlites. They can be fired by the speedlites too. I will have a 4 flash setup. I'll probably won't need my DIY softbox anymore, but I will yet have to see the supplied Elinchrom softbox reach the same level of softness provided by my own.