Even the ones , TO-220, have an internal temperature higher than the outside of the case. And the thin plastic on the back of the TO-220F(P) would or should be very similar to the plastic or mica insulator that is needed with a regular TO-220. Just do like the others said and you should be good.Issue is back side of mosfet, which is to be connected to heat sink is not metal, its some plastic type, so this I fear will that be able to transfer heat.