Yes, the ceiling in question is a vaulted ceiling on a single-story extension, so there is 150mm of insulation there. Would it be OK to make a "X" cut in this and squeeze it partially to the sides?
It'll leave you with an uninsulated spot - so when it's cold outside, you'll have a cold spot on the ceiling. A cold spot will attract condensation, and if the hole isn't sealed then warm moist air will make it's way through and the condensation will rot your structure.
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