I see now that there are small vent holes on the top (4 holes) and this must be where the unit is supposed to vent from.
When I was gifted with the unit I read about the issues with temperature control so I made sure the calibrate the unit’s thermometer. It seems to reasonably hold a stable temperature at standard room temps.
Yes the incubator is plastic. I have a couple of eggs in it left from a brooding hen who didn’t wait for them to hatch. After they’ve had time to hatch and my incubator frees up, I’ll make the modifications. Based on the incubator designs I’ve studied since this thread, I’m all the more convinced this incubator was designed with poor ventilation and that would be the likely reason for the bad experiences mentioned in the Amazon reviews across the various Janoel 10 clones.