After connecting the other side of the fabric:

I could finally send some current through it. The measured resistance is about 0.6 Ohm ( a bit higher than calculated). With 5 V applied:

With 12 V applied it should have an output of about 212 Watt. I can now add a thermocouple, the aluminium plate and insulation.

(Edit: I forgot to mention that it does get warm - even though it's heating a large granite plate!)