Entrance hole protective covers? Yes? No?

Some fantastic info in those links, Benn! Lots of food for thought there - the things that jump out immediately are the very strong preference for a deeper nest box (higher hole) and the preference for woodcrete which may or may not be because that box was smaller (11cm diameter) and - they speculate, at least - less light inside. Hmmm! Redesigns coming up for next year I think. Also the idea that colour can make a big difference is something I had not thought of at all (green is better than brown, apparently).
I wonder if there are any more studies like this around?