The new Seaways Ships-in-Scale magazine (one of FSM's competitors) has an article which includes making cowl vents from brass tube & soldering them up. Of course it was in a larger scale.
Check the Toms Modelworks website. He had an aftermarket resin set to correct some of the deficiencies of the kit. I'm not sure of the contents. Even though Tom has just passed away the family says they will try to keep the business going.
What size & shape cowl do you need. You may find some white metal ones in some of the ship fitting dealers such as BlueJacket Shipcrafters or Loyalhanna Dockyard.
If all else fails, get me a good dimensioned picture. I have a supply of left over resin kit parts and there are several types & sizes of cowls in the stash.