The halyard will slip through the Harken cleat (big and small model) sooner or later. The Ronstan cleat will hold the line for sure. As the jaws are upside down there is no issue in digging into the cleat. The only issue is that the plastic guide might wear over time. Anyway I prefer to have the spin stay up at all times.

If you really want to use the harken cleat you can buy a new swiftcord halyard and splice a piece of rope where the rope should go in the cleat. This makes a 5 or 6mm at that part making the cleat grip better.

Gill