You've been asking and NPO-AEG was nice enough to provide us with the guide to modify your third party AK AEG magazines to fit your NPO-AEG AK-12 AEG!
To fit other brands of standard AK AEG magazines, one will need a filing too or a dremel
1. Initially, the magazine front is filed down mildly on both sides.
So it could fit the front part of the magazine well.
2. Next, the magazine is tried to fit in.
If the magazine does not fit, then it will require further filing on the sides.
3. The magazine is tried for fitment for the third time.
If the magazine does not fit, it is necessary to file the bottom of the back locking latch.
The trial and error are performed in a row.
Step 1 is necessary for all magazines.
Perform Step 2 and Step 3 as needed.