Well, that is a new one on me. Is that how it came from the factory? Or have you sent it in for repairs? Either way, that is a seriously odd way to deal with the funnel.
I don’t really understand the point of the rectangular opening shown in your picture, unless it is meant somehow to ensure smoke fluid flows through the funnel into the smoke unit. But the dynamo smoke would inevitably escape through there.
To seal it I think I’d cut a piece of thin sheet metal to fit over it and fix it down with gasket sealant, which resists solvents and oil, or JB Weld epoxy. I have used JB Weld to plug holes in the top of smoke units as it is meant to be heat and solvent-proof. So far I have had no instance of that failing but my engines do not get a lot of run time.
