I know that Gaylord offers custom box sizes, and I’m sure that other archival supply companies do the same. We’re working with them right now for a very specific custom box, but haven’t received the price quote y et. They claim that there is no extra charge and no minimum order for custom boxes. Just taking a quick look at their most recent catalog, they offer at least flip-top, drop-front oversize print, clamshell, and newspaper boxes all in custom sizes. It may be worth getting a quote to get just what you need. This way, you could also order boxes that were the same size as ones that you already used in your collection, too. Good luck!