I have seen plastic flat Z springs that work in AR magazines. However they wear out and take a “set” much faster than metal ones. I see them as a temporary solution at best.
Perhaps a positive image for a cast? Melt down some brass and pour into a mold? Wish I had the time to play with this.
I am sure that round springs would be much more difficult.
I think that anyone in a state with existing or easily-foreseeable gun control should be loading up on the actual springs that make their factory mags work. Again, they’re cheap and not against any law to buy or own (not yet, anyway). The boxes and the followers can easily be made out of plastic or crappy, sintered, metal by a 3-D printer. Of course, I’m in Texas and we don’t ban fun stuff or 2A stuff down here...and I have plenty of mags, as does everyone that I know who owns any semi-auto of any type.
Still, 10 years on (and maybe less), the then-new 3-D printers will spew out wonderous things like cookies at the press of a button...and I cannot wait.