(contents revised Feb 2015)
First of all, open the box! You'll find 4 screws accessibles from the bottom side; they're hidden but they're there, just use a small flat-head scewdriver to uncover the four corners.
Locate the temperature sensor (see the image), and drill a hole in the plastic cover of the box just over the sensor (yes, unfortunately very near our little friend christopher - the Lunatico man); any metal or wood drill will do, some 7mm diameter.
Make sure to drill the cover removed from the rest of the Seletek! Excercise care, use gloves and with the plastic cover firmly held to the table - drilling plastic can be dangerous!
For best results, use some foam around the sensor to isolate from the rest of the board. There should be free air flow from the sensor to the just made hole.
For the Armadillo and Platypus controllers, a similar modification can be done, but this time the temperature sensor has been placed more smartly, more isolated from its surroundings: