Use a 4 1/2 inch grinder with a smooth diamond blade the chip free one.
Cut a shoulder about 1 inch deep all around. Make that round hole a square
cut some lines about 1 inch apart parallel with the edge of that square kinda like slicing the inside of that square in 1 inch slices.
Make sure that all these cuts are about the same depth
To do that you can spray paint your blade, the side that you see when you cut
and let it dry a bit and then when you make the first cut the paint will
be removed by the concrete and voila you have a depth marker.
Next time you cut you will see how deep you have to go with the grinder running.
After you done cutting hit the slices perpendicular with hammer and chisel
or a chipping hammer.
They will come out easy and will leave you a nice same depth hole
Then fill with hydraulic cement if wet or other pool rated cement if dry
hope this helps