For this DIY how to make hex thermistor for 3D printers you need:

Hex spacer, any you can find (Do not use aluminum it might get fused with hot end, yes I my older version of this article I use to say aluminum but got problem with it recently)
Thermo-resistor NTC type 100K – 104GTA-2 – Specs are here: http://www.mouser.com/ds/2/362/gtthermistor-6057.pdf
This thermistor is pretty good, if not the best for this job, it works in -50C – 300C range, it very precise and very small, Mouser
Teflon coated wire, ebay
Teflon sleeve, ebay
Muffler cement, I used high temperature one, I got cheapest I found in CanadianTire
JST connector with leads, ebay
Some heatshrink

Here are the ingredients you need to have:

Step by step process:

– Cut thermistor leads at 4mm from the themistor, they should be pretty short
– Solder thermistor and Teflon coated wires, preferably use high temperature solder, >240C
– Put teflon sleeves over soldered thermistor and wire joints.
– Use multimeter to measure resistance it should be 125kOhm at around 20C room temperature.
– Stuff muffler cement into hex spacer, try to fill in all available space inside the hex spacer, I used tiny screwdriver and stuffed as much as possible muffle cement into hex spacer.
– Stick thermistor inside of hex spacer, wipe away remains of cement with you fingers
– Cleanup all oozing cement, make sure it stays secure inside the spacer
– Leave it for 24Hr

– After it dried and thermistor is secure, solder leads of JST connector, polarity doesnt matter.
– Use multimeter to measure resistance it should be 125kOhm at around 20C room temperature.

– Attach it to the extruder block
– VOILA !

And yes, dont forget to calibrate thermistor once you put all together.