Would there be any benefit to mounting the sensors to the extruder so that they would move with it and get readings across the print surface? Maybe mount them an inch or so away from the nozzle?
I doubt if cooling is the problem as much as uneven cooling is, though. At least that's what causes stress in castings, especially in materials that don't conduct heat very well.
As I recall, cooling large glass telescope mirrors is a process that must be carefully controlled over a period of years to prevent stress distortion.