I took out the four tiny screws closest to the center, removed the aluminum C/Y lens mount plate, cut off the C/Y bayonet flange, ground the C/Y plate and Sigma plate to fit together--initially the C/Y plate is too wide at one step in the Sigma plate back, sanded the mount surface, sanded the back of the Sigma plate, then epoxied the two plates together*
Screw C/Y plate back on lens.
TODO: drill and thread holes through both plates, screw them together from the inside or C/Y plate side with short screws. (That is how I do this kind now, later)
The nasty empty holes in the Sigma plate are from a botched operation with a different lens.
* Addendum: Don't epoxy a Sigma plate to a C/Y plate. I did much better with this type where the old C/Y mount screws inside the Sigma plate hold the C/Y plate on the lens, and the Sigma plate is only screwed to the old C/Y plate--not epoxied as in this first version. See this gallery, Zeiss 28mm f2.8 for a better way of fastening a Sigma plate to a C/Y plate which is fastened to the lens.