Yes that reinforcement weld exactly what I was thinking of doing. That's interesting on the MAC-11 mags. My buddy's MK-760 has the same prob.
These mags are very versatile! They are 7.5mm mags, and therefore take the longer rounds. Not sure if they'll take .30-06 but I doubt it; I just haven't checked. So any round that's .308 sized or larger could work in them. I wouldn't go shorter than .308 since the followers would tip.