It depends on whether you are horizontal or vertical. Your supplementary note is 297X840, which should be a vertical book, that is, the size of the book is 420X297, which should be 420-sided binding. So your cover paper should be 297 x (the thickness of 840 books). This paper is basically not ready-made. You can ask your friends in the printing factory to cut it for you with large paper, which is too simple for the printing factory.