Repair Ceiling Hole for Round old work ceiling box
Likely too late to be of use to OP, but for others who may come looking. 


Look what's up there? Lath? Joists?
Cut a nice rectangular wallboard patch larger than the hole. Trace its outline on the ceiling.
Remove plaster within the outline so patch can fit in snugly. Ensure that opening is wide enough to access a joist to which box can now be affixed.
Install box.
Cut opening in patch to allow sufficient access into box.
Attach patch - now a rectangular frame - to ceiling structure.
Apply joint compound and finish


I'm in the midst of a repair where I'm stripping plaster off lath and will then glue and screw my patch in place. I can update once I'm done.