On a whim I decided to see how well a 3.2mm RTD fit in the sightglass screw hole (M4x.7 screw). It fits well but the sightglass won't stay put with just the one screw holding it in place. I tried using a washer on the one side with no luck.

Anybody have any clever idea on how to make this work temporarily? I'd like to test this out before possibly going the drilled out vented screw + thin thermocouple route.

Maybe you could use a Tinnerman fastener.