dont' bother cause it won't be really accurate anyways?
In reality, its still an external reading, and is still subject to problems related to all external readings....
If you really want to do this, use thermal tape or soemthign similiar to attach it to the CPU pcb... and absolutely get the tip of the thermistor as close (preferably touching) the cpu core.
Remember that temps do vary along different sides of the cpu core, etc
Mike