I'm stumped at what is causing the deaths, just think that they are not getting back in to the brood chamber in time for nightfall and then just freezing ????
I have the plastic excluder and have #8 floor mesh. Haven't counted lately how many frames in the brood chamber but somewhere around 18-20 and they are pretty much full to the hilt. My hive is about 1200mm long in total. The hive is very active, just not the super. Guess that it shows that bees don't prefer to work horizontally.
On my flow cedar double brood hive which is bursting at the seems, has the flow super on now for nearly 8 weeks and they took to the plastic frames immediately. Did an inspection yesterday and it is only about 50% capped (if that). People at my local bee club yesterday all complained that there is a lack of honey this summer around western Sydney/Blue Mtns, most likely cause of the long dry spell but now with recent rain, things should pick up again.