I prefer to use heavy lumber, rather than old business cards, in my coin boxes. Yes, the
good old 2"x4" does the trick.
Plug those empty spaces in your coin box with 2"x2"
wood spacers. Split a 2x4 on a borrowed table saw, or you can find 2"x2" lumber at your hardware store. Just as 2x4 lumber isn't actually two inches by four inches (really!), you'll have
wood that's about 1.5" by 1.5".
Still, that fits your coin box nicely.
I always end up with scrap lumber, so I clean it up and trim to various lengths. The ones that are 2" long fit especially well.