I carefully splitted the chain with a good quality bicycle tool, dressed a bit a link I took off the chain and reused it as a master link. A bit tricky but doable, rivet the pins a bit after. It was a tip given to me by a shop owner and it seems to work
four racing seasons @ 10,000 RPM and never had the chain split on me.
That said, these master links with flat sideplates do exist, check someone like bikebandit of dennis kirk