Heh, that's why I asked about that early on.
Mica is a good heat conductor and an electrical insulator. Available in sheets from your handy dandy electronics supply house.
You're going to find in very hard to accurately take a resistance measurement with your multimeter. Not only are £5 meters really bad below ~10 ohms, but the resistance at the probe contact is going to be big factor (along with probe placement differences). And to make things worse, the resistance will change will temperature. If you can't get a number for the bulk resistivity from the supplier, you should attach leads to a peice and measure the voltage and current.