What is that round silver thing, a door knob? A real close-up pic would help.
What comes to my mind to close up such an "Oops" hole is to install a vent plug. Then it serves a purpose, allowing air to circulate between the under-bed and cabin to control trapped humidity that might accumulate under the bed. Just as important, people will think is was designed to be that way, not seen as someone's mistake.
Vent plugs can be bought at home improvement centers and hardware stores.
Here are a couple varieties of plastic vent plugs. They come in many diameters, colors, materials & styles.
