My aftermarket receiver cover with inexpensive red dot is easy to use and sufficiently accurate for my purposes. It did require a bit of file work at the front, but has a secure fit and doesn't interfere with loading. For every one of those mounted, there was an OEM cover likely thrown in a drawer or tossed.
If the SKS design had incorporated a notch in the ring ahead of the receiver cover nose, it would have been much more solidly mounted. That would not have been consistent with Soviet production goals, however (perfect is the enemy of good enough). Optic use is complicated by the stripper clip loading and vertical ejection, and accuracy of the service round was also "good enough".