The ThickSolid declaration is used to create a "hollowed" out part. Set the offset to the desired wall thickness and specify the faces which should be opened. The join_type can also be specified to one of arc (default), tangent or intersection.

# Created in DeclaraCAD
from declaracad.occ.api import *


enamldef Assembly(Part):
    Axis:
        pass

    ThickSolid:
        offset = -1
        faces = [
            self.children[0].topology.faces[1]
        ]
        Fillet:
            radius = 2
            operations = [
                box.topology.edges[i]
                for i in (0, 2, 4, 6)
            ]
            Box: box:
                position = (-dx/2, -dy/2)
                dx = 10
                dy = 16
                dz = 8

Using a positive offset expands outwards Positive offset

Using a negative offset expands inwards Negative offset