CMC Power Mag bodies, Metalform convex steel followers, IMSI springs and Wilson steel or aluminum bases may be the best (7 round) "Frankenstein magazine" to be had.
You can buy the Metalform followers, IMSI springs, and Wilson bases as parts, but you have to buy the whole Power Mag to get just the mag body.
The CMC Shooting Star mag bodies are probably as good, but of course the base is welded.
The Wilson mag bodies are also almost as good as the Power Mag bodies, maybe just a bit thinner steel.
The Metalform mag bodies AREN'T as good, and dent quite easily.
The Wolff mags look good, I've got three, but one is showing some spreading at the top, so they may need a little more development.