Ok, Here is your answer. This has been a fairly close secret that us "pros" have kept to ourselves....
This is how to add a removeable mag to your SKS......
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.........It's called an AK-47.
They come with a removeable mag, and are considered very reliable. You can pick up a reliable example for around 300-350 right now. That is only 2 SKS's.
I'm sorry for being a smart ass. I'm just trying to save you from what I've already done and realized. An SKS is a simple, fantasitic rifle, that holds 10rds in a compact mag. It is very reliable when someone doesn't try to make it into an AK.
Keep that SKS as it is. Buy an AK. I know, you can't afford one; no problem, shoot the hell out of the SKS, and learn to use stripper clips. They are fast and reliable. Save your money, and buy an AK when you can, but don't sell the SKS. My collection of them just seems to grow, and they go out to the range as much or more than my AK.