Does anyone make the mauser spec mag box's? If not why?

yes, nearly everyone, will make them .. for example, 505 gibbs requires a specific mag .. but the 264/7rem/300win/338/458 all use the same geometery

blackburn does, or did, make specific ones

if you can wait and pay, i am certain any maker will get around to what you need .. though 1 off production costs big bucks.

That is not all the is required to make a rifle feed a cartridge it was never chambered for.
Action work is also needed. Feed rails need to be cut.

But cartridges are basically classified in groups that sort them by size.

i.e .308 length, 30-06 length, Magnum etc.

feeding has more to do with the follower and feed rails then the size of the magazine and it is easier just to build on Mag that will fit a group of cartridges

Duane Wiebe - Sound Metal Products does as well.