I bought some correx nuc boxes from Easy*bee.
I believe, From previous reports that company sells them. Others do too.
They come in one very large piece, and take quite some working out to fold correctly
Just consider some of the elaborate packaging for the up and coming (probably here already) easter egg market. Often one piece packaging. Not designed by someone in a backyard bee house or hen house. Maybe not so large, but possibly more complex.
Carton manufacturers - if they were unable to supply what the customer wants, they would not be in business long. Perhaps I should have qualified my statement slightly and put 'large' in there some place or places. Yes, there may be some specialists who concentrate on a particular niche market, but as you say, it is only one piece of sheet material. It is only a box. I have seen some very complex mind-bending product packages in other parts of industry. Some are far more ingenious than the 5 frame nuc box, I can assure you.
RAB