You could use a typical zigbee leak sensor, as those also monitor temperature. You would need a small styrofoam box to cover it to avoid ambient air temperature impacting the readings. Not exactly a contact temperature measurement, but that is the smallest battery powered device that comes to mind. Although, probably not much different than your ST Multi Sensor.
If you’re a tinkerer, you could easily build a temperature sensor using my ST_Anything software, running on a small ESP8266 microcontroller, and connected to a DS18B20 temperature sensor (or similar.) Let me know if you like to try this path, as I’d be happy to assist you.