I was slightly surprised to see this image captured today. The bluetit didn't go into the box then, but a short while later it was back again.
This is the sideways view. If you wonder why you cannot see the floor of the box, remember that the nesting material will be 4 to 6 cm deep by the time the chick are hatched and so the floor will be well covered then anyway. The camera is aimed at where the bottom of the nest cup is expected to be at that time!