The only aftermarket magazines that I've found that approach the reliability of the standard box are the aforementioned "Red Star" 20-rd fixed boxes. While they generally take some fitting/tuning for the individual carbine, the job is easy and intuitive. Basically, one simply matches the configuration of the feed path presented in the stock magazine by some judicious squeezing and such. It's worked for me on two makes of SKS.
IIRC, the device itself was developed by the ChiCom military for a short-lived select-fire varient of the carbine issued to some units in the early '60s before the AKM became widely available. I have seen archived photographs of troops armed with these, and of examples of the weapon, but I've never seen one "in the flesh".
If one simply must have an SKS-type carbine with 30-rd capacity, one of the versions factory-modified to accept AKM magazines imported by Navy Arms et al several years back would be the best way to go, IMO. The major problem is that there weren't nearly as many of them imported and demand has driven the prices up near what a ROTM AK varient brings.
The twenty-round fixed box doesn't affect the handling characteristics of the weapon nearly as much as the "detachables", operates the bolt stop reliably (something many "high-caps" won't), and recharges in the same manner with the same strippers as the stock unit.
FWIW, they're still out there. I see them fairly often at shows and they are frequently available from folks like The Real Hawkeye who either expected them to be a direct "drop-in" and took it back out after a couple of malfs, or never got around to installing them.