My new ones seemed slightly smaller than the holes, so fit easily, but do not seal completely. The engine should not produce fumes, as the breathing system should be sealed, as is the exhaust, but if any fumes are produced, it will get into the cabin. The cause should then be addressed immediately, not worry too much about the duckbills preventing the effect.
The duckbills usually perform another job, to direct water away from the bulkhead, so it drains to the ground, not soaking into sound insulation, or sitting in traps to create rust. (Or for some Puntos, not into the alternator) So leaving them out altogether could lead to problems elsewhere.