I use the thermasil ones. you use insulators to get good thermal transfer. The heatsinks
are isolated from everything, so you don't even need insulators, but if you use thermasil
or other conforming ones you get better thermal transfer. Even if there is a short between
the case of the transistor and the heatsink its ok, as long as you don't touch the heatsink
I'm pretty sure this is the one I used mouser part number
567-175-6-310P