Well, mounting a substitute mag permanent-style requires that the hostile LE person who's trying to arrest you for your illegal rifle KNOWS or is WILLING TO LISTEN TO the explanation of the magazine.
For the most part, it's "All non-stock magazines are interchangeable. This gun's been illegally modified." Next stop, the Graybar Hotel, a confiscated rifle, and a day in court plus a stack of legal fees.
Also, I've been made aware of at least one SKS that had a high-capacity
FIXED magazine that was rendered "interchangeable" by a swift palm-slap on the bottom/back of the mag. AK mags (The usual choice.) are made outta sheetmetal, and are usually pretty thin. Because his SKS had a "banana clip" on it instead of the stock mag, (Or even one of those 20-round factory mags that're a star-embossed triangle.) a friend of mine wound up in jail for six months with a felony comviction, lost his gun rights, and his gun got chopped.
Because a cop could swat a cheap pseudo-permanent mag modification out of his gun. The magazine bent, but the only reason the cop even had an issue with it was because it wasn't a stock SKS, and most cops know the difference.
So, for real, look hard for one of those double-size stock mags. I think they're for the Chinese guns, specifically.
And on a lighter note, think hard about getting a Fibreforce Dragunov-style stock for the gun. It winds up looking cool, if you're into that exotic weapon thing, but mostly it CHANGES THE FEEL OF THE RIFLE COMPLETELY!
The pull-length is different, the cheek-weld's different, it's got a pistol grip, and it absorbs MOST of the "snappy" part of the SKS's recoil, something you know about with that factory wooden stock..
It becomes a whole diferrent gun. For all of about $50-$60. Well worth it if it suits your tastes. My bud absolutely loves his, (Not the jail-time dude.) and even I think it's a blast, and I hate pistol-grip assault-weapony, black-rifle-flavored guns.
It's the best for hopping an antifreeze jug back and forth on the range.