I have put in NextCloud (Contributor Resources > Cover7 folder) a
template (ODG) for 6x9-inch books printed at Lulu. Link below. This
template shows the bleed and trim areas for the covers. (The width of
the spine varies, of course, with the number of pages in the book.)
Ideally, in addition to producing covers for the PDF version of the
books, someone would produce front and back covers set to the correct
size for the print edition, with the correct proportions and the extra
margin for bleed and trim. I (or whoever) can then drop those into the
wrap-cover template and add the book name on the spine. I've been
doing this in a sloppy manner (by distorting the A4 cover) for 15
years, and will continue to do so if necessary.
https://nextcloud.documentfoundation.org/s/HTtTreGR4XN8Y4d
Jean