Hallo why you dont think out of the box.
1: What do you whish of your sensor.
2: whish you read all temp?
3: Or do Which a critical temp to now it to High temp?
Of its nr:3 you can use a old fasion termostat whit gas senor whish only on / OFF switch
In your saunabox the Electrican box have a old metallsensor whish gas in a small tube to registret to High temp.
Of you buy a old On/Switch and the you use your Fibaro wire output to the Old On / OFF Switch, so your Homey now when the temp is to high.
Good Luck
Maybe this termostat
You can see Where the sauna
Original temp-sensor its place.
And do not have no Cable on the inside Top of sauna Roof. It the High temp up and not so High on the downside. Why not Ask a certifiering Elecrican he now how to do in your sauna. 
