This model was quite difficult to design, as the two sides of surfaces made with PHiZZ modules differ a lot (due to the presence of “bumps” where units join) and the Klein bottle is a one-sided surface. Thus, there must be a place where units pointing out in one direction join those pointing in the other direction.
I tested several different solutions before choosing the simplest and most elegant one. Last image shows a close-up of the bottle’s mouth, where units with two different orientations are connected. One of the earlier attempts resulted in creating the atomic mushroom cloud model. The last picture in top row shows a partially assembled Klein bottle, with inner parts of the “neck” visible.