That's not really a "speed ring" in the conventional sense of the term...that is a retaining ring. A traditional "speed ring" has four holes in it for the rods that hold the box in its shape. I am assuming that the Interfit softbox itself has a ring or collar, which has four holes in it, into which the rods are inserted, and that the above-linked ring goes inside the box, and through that ring, and then the lugs (the cut-outs) go into the lug receptacles inside the flash unit.
What you need is a traditional "speed ring". One that can be used on multiple systems. You need a generic, speed ring for use with speedlights.