The marina at Troon has an excellent slip that is well sheltered within the marina. Can be rough outside the Harbour but probably no more so than Ardrossan.
About the same distance as Largs the other way, again the marina has an excellent slip.
If it has to be free, there's a public slip immediately adjacent to the marina, probably only good for the top couple of hours of the tide but parking won't be an issue and it's a bit more sheltered than the RLNI slip in the town. Just watch the rocks on the way in.