When I was a kid in the early '70s I had these 3D molds of little characters that I’d pour a pink mixture into. After the mold set I’d remove it so the figure could dy - and shrink. They could then be painted. I think later editions replaced the pink mixture with blue.
I can’t remember what they were called, though. Shrinky Dinks are flat and I think came a bit later. Anyone know?