Shell
The Shell menu can be found with the [Shell...]
button in the main General Tool menu. This operation creates blocks around the area's sides.
The block selection menu at the start of the menu starts the Shell operation once you select a block with it.
Inner Block
The [Inner Block]
option lets you select a block to fill the inside of the area with. Once this option is on, you can use the button again to turn it off.
Side Settings
The [Side Settings...]
button opens a menu that lets you change how many blocks thick each side is, as well as what sides to place.