Before hicap magazines became available from the factory I fixed a lot of aftermarket mags. The usual issue is that the lug on the back is too high, allowing the magazine to drop below the feeding point. The fix is to build up the bottom of the lug by welding to make it longer. Then using a file remove a little and see if it fits. This can take many tries until it finally seats properly, but once done it will work. If you're not good at welding, have a welder do it. The best welders for magazines are the muffler and exhaust guys who weld thin materials everyday. They'll usually do it for a few bucks and the rest is just your time to fit it.
With factory mags available and affordable it only makes sense to do this if you have a lot of old mags. Back in the day I paid $60 or better, each, for LEO only 20 round mags. I'm glad those days are over.