the diagonal of a 12" square is 17"
if you have a taping knife of 12-14" you could fill each corner of the hole by starting from wherever you can be in contact with the original ceiling, near the centre, but then pull into the corner.
You might have to do opposite corners and wait for it to dry.
Once dry, the next two corners, then dry, then the centre.
UPDATE: thanks for all the suggestions. I used my wide taping knife to build this up. It probably needs a couple more coats to finish off but definitely making progress. I’ve added the initial picture along with the second coat. Will post an update once complete.
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