To go Single Shot, you should just be able to use the Score High Follower for the 243 Mag Box you have now.
For Repeater I would say just get the 223 box and follower, they aren't that much and should take a lot less "tweaking" (if needed) to get the 223AI to feed from rather than a modified 243 box that "might work".
All the above and a $0.98 will get you a dollar cup of coffee.
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