I just had an idea. I have this happen with S3D all the time unintentionally. In your drawing, set the wall thickness of the sides that you want open to be too thin to print with your nozzle size. If you are using a .4mm nozzle, S3D sets the layer width to be 0.48mm. If you make your wall .2mm thick, S3D won't print it. Experiment with the "advanced" page "external thin wall" setting to see if one of them will make this trick work.