Mica insulators of the size and type I need are getting hard to find.
So I bought some of these http://www.jaycar.com.au/Electromechanical-Components/Heatsinks-&-Accessories/Thermal-Compounds-&-Adhesives/TO-3-Silicon-Rubber-Transistor-Insulator---Pk-20/p/HP1150
I know they are supposed to be...