Transistor Insulator.

here's a good chart of thermal conductivity for various materials. PTFE is shown as 0.25, which is 10 times better than air (0.025), but mica is 0.71, and aluminum oxide is 30 W/(m*C) which is by far the highest of any electrical insulator in the chart.

https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

Ok. So potato has twice the thermal conductivity of ptfe! Don't recommend that, though . Polythene looks the best of the commonly-available plastics.