I use two duckbill Cobray 30 round mags in my SKS with total reliability. In my opinion, the polymer mags are better by far than steel mags primarily because you can work with them, and usually have to to get them to function without problems.
It's as simple as getting a fine file and filing down the feedlips and sometimes the area that activates the bolt hold-open. Cycle the bolt, see what's interfering, file a little bit, check operation again.
Sometimes polymer mags have more "feed lips" than they need. You can file more 'gap' between the lips and actually improve performance. But it is important to only file off a minute amount of material at a time and check operation again or else you'll render the mag into a useless pile of plastic. Make sure you file the lips 'square' and straight, not crooked and angled.
And by "check operation" I mean 'cycle the bolt'. It's highly advisable to use Snap Caps while checking feed and bolt hold-open. But they are expensive at $15 or $20 for five.
I must admit that I used live ammo.......so if that's the case for you, for crying out loud keep the muzzle pointed in a safe direction away from other people, preferably out in a rural area. Go to a buddy's house if you have to. A negligent discharge can go through two or three walls and get you into a lot of trouble.