Good point, there's a basic mis-match there between the design of the plastic feeder (French) and the usual National cover board (UK). Is there scope for one of the larger manufacturers to commission a matching hole version? A quick/cheap alternative in the meantime might be a blank cut from something thin (ice cream tub lid?) to cover the void leaving only a circular hole. Could be attached to the board, or maybe easier taped to the feeder - peel off and the cavity only needs a quick rinse end of season. Making a couple to try is on the winter list.