A while ago I bought a main's powered flash and softbox from ebay. The flash is one of those things that I'm unlikely to use very often and will stay at home, but I have been thinking of ways of leveraging some more value out of the softbox by mounting it to a stand and mounting a speedlight to it.
I had a couple of strobists around to my house last weekend and they figured out how to mount it to the stand. The speedring for the softbox is rather large. It uses four screws which can be tightened to attach the mains powered flash. If you turn one of these around and screw it in tight, you can put it into the top of a standard umbrella mount.
With that problem solved all thats needed is to mount the flash. This can be achieved by using a clamp with a ball head, which you can clamp to the speedring and then attach the flash to the ball head using and adaptor. You could use the foot that the flash came with. In this case I'm using a sonia all purpose adaptor and the clamp I got from Maplins
The softbox is still slow to put up and bulky to pack compared to an umbrella but when I get a bag with wheels, is going to be a very useful addition to my kit.