No mesh loaded
How it works: the inner wall is the set of points lying exactly your wall thickness inside the surface, found on a distance field rather than by pushing vertices inward. That is what makes it hold up on detailed models — the cavity cannot fold through itself, and anywhere the model is thinner than twice the wall it simply stays solid instead of turning inside out. The cavity you get is the largest one that thickness allows. The export is a single closed solid, outer skin plus inverted cavity, with drainage holes cut through the wall by a real boolean subtraction, and the file is checked for holes before you download it. Wall thickness and hole size default to values that suit the model you load.