Phillip -- When I need to label a book with a call number in the archives, I will use the regular call number label, but stick it to an acid free strip that I put in the top of the book (with the call number sticking out). Something like this: https://www.universityproducts.com/rare-book-identa-strips.html

If the book is fragile enough to need to be in a phase box, I'll stick the label to the box.

Alternatively, you could also just write the call number on the strip or box in pencil.