The surface must be smooth and not curved in the area of the suction cup. I don't know how much such a SUP is inflated. We once wanted to attach the suction cup to a professional rubber boat, such as those used in transport and rescue. Although these boats are pumped rock-hard, the suction cup does not last long. This is because the surface inside the suction cup gives way, releasing the vacuum. On a rigid window or sheet metal, however, it holds perfectly.

Thank you very much! That's what I expected. :)