Don't!
Unlike other units, the CS's have insulation in the walls. If you drill a hole you risk affecting the integrity of the machine.
A more simple solution is to buy a remote probe thermometer, like a Polder, and run the sensor probe & cable down through the vent hole. The accuracy of these units is much better than the large dial thermometers.
You don't need to monitor the temp of the unit like you do in offsets in order to maintain a certain temp. Just set the dial and it will "average" the temp you set. Just like an offset, the temp will vary some, up and down, but the average will be what you want.
The only time I use a probe is 1) insert in what I cook and 2) about once a year to check the temp dial and see that it's working.
Hope that helps.
Smokin'