I find the magazine thing kind of funny. Even if we can't quite print guns yet, a magazine of any reasonable capacity ought not to be too hard. I'd be a little surprised if it hasn't been done already.
It's easier to manufacture magazines using regular metalworking process, though. I'd possibly 3d print the follower, but buy great CS springs (potentially scavenge them from 10rd magazines, potentially also with the followers), and then either fix 10rd magazine bodies or just bend some sheet aluminum into the right shape.
The big problem for common weapons like the AR-15 using STANAG 20/30rd magazines is "real" USGI aluminum magazines are still widely available, and cost about $8/ea usually (they just spiked, but a lot of people have tens or hundreds already). The best magazines, Magpul's PMags, are $125 for $10. By the time you 3d print, add springs, etc., it's not worth it.
There's not a whole lot of stress on a magazine (certainly compared to a barrel), but they are subjected to wear, and need to smoothly insert. There are some highly dimensionally sensitive areas as well as flexibility sensitive parts (feed lips) which also change with time. Even the stainless steel magazines basically suck by comparison to Magpul or USGI aluminum, and the Thermold plastic magazines are also lame. The only polymer mags other than pmags that I don't hate are the Lancers.
There probably would be a market in custom or semi-custom manufacture of magazines for more obscure guns, though. Or, if 10rd magazines were completely banned too, and somehow the huge supply of 10-30rd magazines were confiscated.
I would be unwilling to rely on one that hadn't been machined, assuming it would jam or so. I don't think the idea of a ban is so much about making them impossible to get as making them hard to get casually. As far as I can see the big prize for the administration would be closing the gun show loophole and dealing with the straw purchaser problem (both of which seem like reasonable things to me).
