Hi.
Maximum cooling and as minimum electrical conductivity is prefered. I'm planning to use it for a very simple power supply, trafo + rectifier.
Input voltage will be in the 230V range, so the absolute most important feature is that the material does not conduct current.
[edit]
After comparing prices, I just think I'll abandon the project. I think that making a simple temperature regulator that controlling air cooling (by fan) is cheaper - after all I have all components needed for that.