Update
I printed a “Proof” copy of the book with Amazon KDP with the above cover template, and it did actually print correctly, centered within KDP’s print error. Looks pretty good!
I’ll post a re-usable template in a few weeks when I get around to it, to which I added the page_thickness x number_of_pages from KDP (to calculate the spine thickness appropriately) - the above one you’ll need to type in the spine thickness yourself using KDP’s calculation (which you can find here).