Since a factory polymer magazine for the 450 isn't likely to be materializing anytime soon, I decided to make one. I tried modifying a PMAG, removing the central rib, but ended up destroying it in the process. Not wanting to try my hand at it a second time (mainly due to frustration levels) I searched for other options. I found the Lancer mags and was intrigued. Polymer mag but the benefit of metal feed lips. I ordered a 20 rounded from Brownells. I examined the inner mag profile and was able to 3D print and modify a copy of the common blue snap-in follower found on ebay to work. I wanted to do one better though and actually print a follower that would replace the factory 223 one and better match the inner mag profile. Attached are photos of what I came up with. With the factory spring it will hold 7 rounds, my guess is I can get one or two more by trimming the spring a coil or two. You'll notice that the rounds are staggered within the magazine but have no trouble feeding into the feed lips when emptied with my thumb. I did not put the magazine into a rifle yet to check how they feed when fired (ran out of daylight and Dad duties). I do need to sand down a few of the faces of the follower for a slightly looser fit though it does function well right now. I may also put a CA glue finish on it to ensure smooth surface contact (trick I use for 3D printed parts in my day job).
I'll report back after a firing test!