As in the subject. I think I've just about tried all the S3D options but nothing seems to work (or I missed something). I attached an image of how it looks. If you put the part against a light source, you can basically see through where the gap is. Notice how perimeters 1 and 2 are connected properly, it is only perimeter 3 that is disconnected. This issue does not occur when only 2 perimeter walls are used.