SST-316 is one of the best bio inert metals but it has a very low thermal conductivity compared to other metals and can be difficult/expensive to machine. In many use cases an Al disk can be used with a very thin (<20um) bio inert coating. Since thermal resistance is proportional to thickness a thin coating will add very little thermal resistance to the heat flow from the skin to the sensor. Also, Al has a much higher thermal conductivity (~220 W/m.K) than SST-316 (16 W/m.k).
The key to maximizing skin temperature measurement accuracy is to make the sensor to skin contact area as large as possible and minimize the thermal mass of the sensor system and its thermal losses to the environment.