The cap is part of the model's intended arrangement and should be restored exactly as instructed. Leaving it out or substituting another object can affect sealing or allow unwanted leakage.
What the observation tells you
The apparent size of the hole does not show the cap's full purpose. A cork, tape or household sealant may be unsuitable for the materials and operating conditions. Check for both internal and external components if the manual describes them.
A useful next step
Keep the freezer empty of vulnerable food until the correct arrangement is confirmed. Identify a missing or damaged cap through the model reference and obtain a compatible part.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.