The universe can be a sealed container without invoking flat earth. If the universe is finite, then it is bounded by space time, which would provide a “seal”.
Even if the universe was infinite and unbounded, gravity still pulls matter into distinct regions of space, leaving other regions mostly empty, creating (partial) vacuum.
Psalm 19:1 is a perfectly good verse on its own, it still does not support flat earth, and there is no need to quote a Nazi that used it.
A vacuum cannot exist without a sealed container- fact.
Ex Nazi scientist and head of NASA, Wernher von Braun has psalm 19:1 on his headstone for some reason...
"The heavens declare the glory of God; and the firmament sheweth his handywork."