The truth is the Shimano boss cover does not work with the Sunrace shift levers. It doesn’t fit.
The issue is the alignment tab on the Sunrace boss cover and corresponding slot on the shifter is narrower than the Shimano tab. There may also be a diameter problem but the real issue is you can’t fit a wider tab into a narrower slot.
You can grind/file the Sunrace boss cover flat, but consider the effort expended if you end up hating these shifters.
John