latticed shutters
蔀 centers on a latticed shutter, a screen or covering made of crossed strips that lets light and air through while providing partial enclosure.
蔀 combines the plant radical 艹 with 部, which likely contributes the sound. The character is associated with latticed shutters, possibly originally made from plant materials.
The grass top 艹 suggests plant material, and 部 below looks like a structured grid. Picture a woven grass lattice used as a shutter to filter light.
For ホウ, imagine a latticed shutter on a traditional Japanese house (hou) that lets in a soft breeze: hou -> ホウ.