Miracle of miracles - it is an exact and very tight fit for the standard reflector that comes with the ERQ heads. I could use a couple of small retaining screws to hold it in place but there really is no need. The fit is that exact.
I have decided to use a Manfrotto Light-tite to take the weight when using my larger Chimera soft box but I'm pretty sure that it wouldn't need even that if a box of around 60 cm x 40 cm (24" x 18") were being used. I have tried the system out and it is pretty good. Ideally the flash tube would sit a bit further forward which would mean that the light spread even more evenly around the inside of the soft box. I'm sure that the various manufacturers will make proper speed rings very soon, but this is a very good compromise for now. It passes my two tests: it is easy to use and it is cheap. I intend to leave the speed ring permanently attached to the reflector on this head. It doesn't compromise the umbrella holder in any way and I have a second head with which I can use other light modifiers.