The readily converted issue is going to be big one in magazines of such a nature. I started printing out (use my 3D printer) .458 Socom followers for my .223 mags. In essence single stack follower that wouldn't allow .223 to feed. The problem I think we'll get into is, what happens when a law enforcement agency can drop thirty loose rounds in the magazine and get a few to fire while clearing jams and such. Then you might have a problem. Law enforcement can be really particular about this nonsense.
For grins and giggles I took three ten rounders I had found at a garage sale and with a hand file and some duct tape plus a little JB Weld, I was able to make a fully functional thirty round mag. It was ugly as sin but it worked for 120rds straight the day I took it to the range.
So you could start building a mag (aluminum sheeting) and before you even cut or bend something, put it int your CNC engraver and engrave .458 Socom 10 Round Magazine, and then run the rest of the engraver for your lips and cut portions and then put it in your mag jig and either use a ton press or do it by hand yourself. Use a bit of metal wire or instrument wire and mandel for your mag springs (will need to heat treat your springs), and cut your base out of the sheet you made your mag out of and do a bit of hand work for the bottom lips which will be helped a great deal by having another jig you premade.
Then when the cops pull you over you'll get to be test case, good luck.