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Go shopping at your local big box store, storage container specialty store, or online to find under-the-bed storage containers that closely match the size of the negative space. Wiggle the containers into the space under your bed. Add heavy items to the containers to prevent them from sliding around. Gather some thick and heavy books that you won't need to use for a while. Create stacks of books that will fit under your bed. The average book is 9 inches (23cm) tall and the average bed is 80 inches (200cm) long. This means you'll need about 8 stacks of books on both sides of your bed, plus more along the foot of your bed. Slide the stacks in one at a time. The number of books in each stack will vary depending on the height of the space under your bed, and the thickness of your books. Shoe boxes are often a great size for this, but you might use food boxes (e.g., cases of almond milk or raisins) or boxes from items you've ordered online. Line the boxes up against the sides of your bed, then wiggle them underneath. Ask around at a local shoe store to see if they have extra boxes. Place heavy items inside the boxes to prevent them from sliding around.
Use storage containers if you can spend a bit of money. Stack books under your bed if you have a lot of them. Block your bed with cardboard boxes for a low-budget choice.