Years ago, in the mob I fly for, the techies used to give the cabin a one minute warning to takeoff which sometimes wasn't enough with some c/c still rushing to get seated. After a little thought and discussion, with all concerned, the cabin manager now checks with all the c/c, via the cabin interphone, that they are all ready for takeoff and then gives the techies a report saying such. The pre-takeoff checklist cannot be completed without this report.
Regards,
BH.