Hollow Layer Specification - CompactMods/CompactCrafting GitHub Wiki

The hollow layer spec is a fluid layer type; it takes a single component and specifies that the entire layer is filled with a single component, only on the surrounding perimeter (the walls). This is useful for hollow, block-like structures such as Compact Machines and empty machine casings.

{
    "type": "compactcrafting:hollow",
    "wall": "C"
}