I use a front panel monitor similar to the digi doc.... the thermistors are very flat and very thin, so I simply superglued the sensor assembly to the surface of the chip with the very tip of the thermistor touching the side of the core.
I suggest you mount the sensor and hold the heatsink and the chip together compressing the pads and make sure the core contact is good before you mount it and fire it up.
My sensor reports chip temps around 40-50c while my onboard monitoring (MBM) reports temps of 32-40c. I tend to beleive the thermistor I installed is much more accurate.