Thanks John. That would be fine, but the outlet has been fitted fairly close to the top of the transom, so there’s only room for a small loop.
I have since seen a heater exhaust outlet with a stainless right-angle bend protruding from it, but can’t find any such outlet on the web. Maybe it’s been a bespoke job.
I’ll keep an eye on things for now. I can always remove the cover if need be, and go back to the wooden bung solution.