I'm not a fan of ambi safeties and considered modifying mine. Short version is I think the right side can be clipped and the frame plug inserted.
From what I recall clipping the right side wing, only the part that sits outside the frame, still allows for proper function of left side only. I didn't think of doing mine until after I had installed new trigger, sear, spring and such and was pretty much fed up with the process.
I don't know of anyone who sells a single side only safety for the M&P - seems like someone should.