Are you willing to drill a hole in your case? If so, let's talk...
I thought about ways to extend the tip, but that just won't happen with the limited knowledge that I have, and making a thermo-conductive element won't work well, as the heat will disipate as it travels toward the mercury bulb.
Okay, so where you drill the hole depends upon where you want the thermo to go. Essentially you will end up with the thermometer poked trough the case and the tip as close to the source you want to get a reading on. Use a rubber o-ring of some type to create a seal and hold the thermo securely. If nothing else, electfical tape wrapped carefully around the thermo will work.
Be careful of spinning fans, mercury and electricity mix well and I believe that that the conductivity increases as the temp of the Mercury increases.
It is a mercury thermo, right
At any rate, enough of the "reading end" of the thermo will have to protrude so that you can, well.... get a reading.
If you are attempting to get surface temps of Heatsinks, RAM modules or other similar surfaces, this will not work as I do not think the thermo will be long enough.
You might want to reconsider using a mercury thermometer...
whatever you try good luck!