If you cannot fing ready made in 18KT might I suggest you try LEX-WAXWax patterns. They sell plastic injections (that you can have cast)in this size #8567 that may work for you.
Otto Frei has them in stock: #6C-132, 3mm Square Hi-tapered withseat. 1- 800- 900-3734. Ask them to send you a findings catalog.
Douglas Zaruba35 N. Market St.Frederick, MD 21701301 695-1107@Douglas_Zaruba