How to secure slab in place on ceiling to apply filler

Electrician moved a light switch for me. He had to cut out the slabs in a few places to fish the cable around to the new location.

The cutouts on the wall are fine but there's one on the ceiling that I'm struggling to fill in.

There's empty space above it so when I apply the filler it pushes the piece of slab up. I can feed the piece of slab to be fairly flush with the ceiling but as soon as I apply the filler it moves up.

I did try to just do the edges bit by bit, to have the filler sort of act like glue to hold it in place before doing the rest. That sort of worked but as soon as I tried to sand it the slab pushed back up.

There's nothing above the cutout so there's nothing that I can screw the slab to. Any suggestions?