The M90 has a fixed 8.5 inch reflector that is only removeable by undoing the screws inside the reflector, and so the vast,vast majority of speedrings labeled "for Speedotron" will simply NOT WORK, because they are for the two-lug Speedotron Universal Mount, which is used on the Brown Line M11 lights, and on ALL the Black Line Lights (102,103,202,206). However, what will work is a special speed ring made for the M90 light units, and which I know
B&H Photo has in its inventory; one night I waded through something like 347 different accessories for Speedotron at the
B&H web site, and on danged near the last page, I found the listing.
If my memory serves me correctly, its a Chimera-branded speed ring. It has about a four inch long metal rod that is screwed into the threaded hole on one side of the CHimera PVC-and-metal square-style speed rings; that aluminum rod is the same diameter as an umbrella shaft, and it allows you to put the rod through the M90's umbrella hole, from the back side of the light unit, and then to tighten the thumbscrew and thus hold the rod in the umbrella shaft receptacle. I have two of these,and they work great!!
Okay, here it is. THIS is the one you want for use with an M90 or MW3U light head.
Chimera Speed Ring for Speedotron 2350 B&H Photo Video
A wide variety of manufacturer's softboxes will fit this ring: Photoflex, Chimera, and loads of e-Bay Chinese models.