The external thread needs to be glued in. To be safe, I made hollows for the spout and used epoxy glue.
If it won't fit on your spout, use a scaling. May need to use an O-ring gasket.
If you don't need to use scaling, instead stopper can also be used a lid from a Cola bottle (PCO1881)  and there is no need to use a gasket (the original cola stopper is soft).