Flushing The Container
As soon as the container arrived Wayne opted to flush the container permanently. He sealed the two main doors permanently and built in a single swing door to provide a single entrance and exit. Sealing the container would need a thorough job, by ensuring that nothing could leak in and out of it. To ensure a one-way entrance, the single door was installed to swing inwards only. Once you see what he did next will make sense of it all.
Two Feet Deeper
Imagine how much of a mess it would have been if Wayne had not dug a hole big enough to fit the container. He made sure to dig at least two feet deeper than the height of the container to ensure a complete fit. Clearly, he had thought his plan through before placing the container. He intentionally left space around the entrance area and about two feet extra space all around the container.