An external flap will be a Rube Goldberg affair - not necessarily a
bad thing if it works, but it would look like poop. How about a
magnetic flap on the inside? That's assuming it's a steel slot. If
not, maybe using some polyurethane or silicone caulk to make your own
gasket and/or swapping out the spring (assuming that it's spring-
loaded and not gravity) for a stronger one.

R

Thanks for the ideas.
It is a steel slot. Any kind of caulking or filling just stops the
flap from returning - as it isn't spring loaded (it returns under
gravity only).