These guys have just come out with some interesting mags...I think I'm going to try one for 450b compatibility:
http://www.etsgroup.us/
I especially want to check them for feed lip spreading when loaded with 450b rounds. Their information seems to indicate that their mag material is designed to resist that:
Our magazines are made from advanced polymers that provide extreme impact resistance so they won’t crack or break when dropped. With superior temperature tolerances our magazines won’t become brittle over time, even when exposed to harsh environments, chemicals, or extreme cold. Its durable construction eliminates creep so the feed lips and body won’t spread when being stored long term, even when fully loaded.
We shall see...