Boxed sets used to come with full-size foam sheets inside. When new they had some resilience. Their purpose was to fill the empty space and put some side pressure on the LPs, to prevent warping, as we do by storing our single LPs vertically side by side. The foam was even used in different thicknesses, depending on the number of LPs and size of the box. That foam lost its spring then rotted long ago, leaving sticky crumbly bits that stick to both sleeves and vinyl — a total PITA. But they once served a purpose and did so effectively. If you're concerned about loose LPs warping in a box, a thin square of styrofoam may serve the same purpose. They say styrofoam cups last 20,000 years in a landfill, so I doubt it will rot away in your collection of Bach Cantatas, as the old foam did. Thin bubble-wrap also works, as do corrugated cardboard squares, and you can build the thickness by adding layers, but it can get a bit OCD. It's a cheap and easy remedy if warping is a concern to you. It isn't to me.
