I don't know if anyone else has tried this, but after fitting the fibreglass sleeving, I fixed my thermistor into the hole in the heat block with Holt's Gun Gum exhaust putty. It only needs a small amount, and after fitting the thermistor in place, the gun gum can be cured very quickly by using a hot-air gun. The end result protects and insulates the fragile thermistor leads, and should withstand high temperatures (up to 700°C, which should be enough!) The only problem is that a can of the stuff costs £3.99 and contains enough for a hundred or so hotends! I had some handy from a vehicle repair, but it could be worth E3D buying a can and selling off small amounts in polybags. Please note that I also ground off some of the edge of the washer to give the thermistor leads a bit more clearance.