Hi Tom,
I was walking the dog through the Auld Kirk churchyard this afternoon. Like the others I thought that this was the obvious choice. But I'm afraid there was no evidence of your monument. It is a small churchyard and any monuments of this size are noticeable. There were a few but none of them yours.
I think Ayr Cemetery would have been a bit too rural when it was opened to have had any such buildings surrounding it.
Andrew