Minecraft wiki entry for Shulker_Box, associated text: When placed on a wall or ceiling, shulker boxes face and open sideways or upside-down, respectively. Shulker boxes cannot be opened if a solid opaque block is on their top surface, similar to a chest. For example, if a shulker box is facing upside-down and a solid opaque block is below it, the shulker box cannot be opened. A shulker box can also be placed using dispensers.