There are 30-round mags designed specifically for the SKS.
My point is that you have to either modify the well, or use an adapter, which, when attached to the box magazine, adds a snout to the front of the mag, or the adapter fits permanently into the well, which then accepts the box mag.
The ak mag does not in itself fit into an sks, unless it is one of the SKS-D models, specifically designed and machined at the factory to receive the ak mag. That rifle will no longer accept the standard sks magazine.