oval
椑 centers on an oval shape, often referring to an oval container or vessel. The meaning is narrow and concrete, with no strong abstract extensions.
椑 combines 木 (tree/wood) with 卑, which likely contributes the sound. The exact historical development is uncertain, but the character is associated with oval wooden objects.
The left side 木 suggests wood, and the right side 卑 looks like a container with a lid. Picture an oval wooden barrel or cask made from tree wood.
For ヘイ, imagine an oval wooden tray used for serving hay to animals: hay -> ヘイ, and the tray is oval like 椑.