OK, it's a bit of a tease, but this is a serious design for a nestbox.
But a nestbox for what species?
As usual, there's that cleverclogs in the third row "Me sir! Me sir! I know, sir!" I wouldn't have been the cleverclogs in the third row, because it would never have occurred to me that this species needed, or would use, nestboxes.
It seems that there are various commercially-available designs, but there is no evidence that they work. Hence the ingenious DIY flowerpot design illustrated above. Here is another:Go on, you've guessed, haven't you? I will give you a clue: they can have white tails, buff tails, orange tails or red tails. Both the DIY designs above, and the whole discussion of how to help this species, can be found here. I can't wait to have a go.