final SimpleBlockMaterialPicker picker = new SimpleBlockMaterialPicker();
box.setPicker(picker);
// basic room
picker.setOuterInnerMaterials(VanillaMaterials.DIRT, VanillaMaterials.AIR);
box.setMinMax(xStart, -1, 0, xEnd, height - 1, depth).toggleIgnoreAir().fill();
box.toggleIgnoreAir();
picker.setOuterMaterial(VanillaMaterials.AIR);
box.setMinMax(xStart, 0, 0, xEnd, 2, depth).fill();
// some 'decoration'
box.setMinMax(xStart, 3, 0, xEnd, height - 1, depth).sphericalFill();
}