Torque specs for a plastic thermostat flange

You have three possibilities:

1. You somehow get the official workshop manual (I have no idea how) and use the official torque (If given)
2. You ask around until you get the "internet quote" of your least mistrust.
3. You do it like most of the people out there, and develop "a feeling" for the needed torque. Yes, with some experience it is possible to do it by feeling, with an acceptable success rate. A possible approach would be to torque down the old piece onto another sacrificial piece, on your workbench, until destruction. So you might get a feeling for the needed torque. Also, have a backup plan in case something breaks.

If you are worried about the bolt coming loose, you can apply some low-strength threadlocker.