Shemirat Shabbat KeHilchata 16:13 brings down that plaster, clay, wax, and the like may not be molded on shabbos. (footnote 36 quotes SA and says it's due to mimareach)
As far as I'm aware, a collection of objects like a magnetic sculpture may be played with on shabbos, since it is comprised of distinct individual pieces - no actual violation of mimareach is taking place.
What about something like playfoam, which functions much like clay but is ultimately comprised of small, distinct foam balls? Is this considered to be like a collection of magnets? Or is it closer to clay, since it is held together by a tacky polymer and not through magnetism?