[Re-Sizing Question]

Apologies if this is the wrong forum, but I need some help with a very specific re-sizing issue on Affinity Photo. I am self-publishing a book through Lulu, and each page needs to be the exact dimensions of Lulu's template, or the publishing tool will flag it for having varying size pages, even though they look the same size.

Is there an easy way to have a layer snap to the exact dimensions of the layer below it (the Lulu) template, to avoid painstakingly clicking-and dragging? I couldn't find a method to do this on Affinity Photo. Any suggestions would be greatly appreciated!