Duct tape should work but the only thing is that when you peel it off for a fresh peace of tape then it will leave a sticky mess under it. The was I would do it is use abit of the sticky thermal grease, the stuff you put inbetween the CPU and Heatsink. The thermal grease is sticky in itself and will conduct heat at the same time, so if you place the probe on a blob of that stuff then it will stick onto it and also be able to give an accurate temp reading. But just try not to move it around too much or the probe could be loose and fall off. But thats the best way I can think of.
Good Luck,
SKY.