I got a couple of small softboxes from Amvona, and wanted to use one of them with my 580EX.
Both softboxes are 22x16", the first one comes with this speedring. Since I don't have a strobe of that type, I modify it by cutting a section of neck off, now it fit my Alien Bees.
Drill a hold through the unused rod holder, fabricate a steel L bracket with the right height and length for the 580EX, bolt tight together.
580EX mounted. A umbrella bracket is used to connect the L bracket and for tilt adjustment.
This Amvona 22x16" softbox is quite light, the Monfretto 3373 stand holds it up with no problem.