I've got half a dozen of the ProMag 5/20 SKS magazines with the additional lever on them. I like them much more than the Tapco ones, but there seems to be only ONE way to install the magazine springs correctly. If I don't reassemble the magazine just right, it pops out 1-2 rounds whenever I go to insert the magazine. I suspect that it would be more reliable if I filed off 2-3mm of the trigger bars that stick out into the mag well, but I really don't want to modify a mint Russian SKS for that.
I didn't bother with grinding down the bolt carrier to insert on a closed bolt. Works just fine inserting with an open bolt.
The ProMags sell for $55-ish at most places, but sometimes they go on sale for $42-d44, I can't remember if it was G4C, FOC, or TNA that had them.
There's also the Hical magazine adapter that converts an SKS to be able to take 7.62x39mm AR magazines, but the kit costs about 50% that of an entire SKS and requires a lot of filing and modification to the SKS.