They slide onto the end of a piece of dimensional lumber, cut slightly shorter than your room, and you then screw an upside-down leveler up until it reaches the ceiling. It then holds the impromptu post in place, while a cap on the bottom protects the floor.
It's obviously not structural, but ought hold the wood in place securely; in Japan they use a similar ceiling-contacting mechanism to prevent bookshelves from toppling over during earthquakes.
They also sell little shelf supports so that you can fashion shallow racks.
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