I'm open to that! I'm a mechanical engineer, and I have just enough materials science knowledge to be dangerous 🤙

The difficulty you'll find with natural rubber is that it can't be reworked after vulcanization, the hardening process, so you'll need a mold for the whole shoe. You could 3D print the positive, use it to make a negative with a temperature resistant material, and then cast and vulcanize the rubber in that negative. Rinse, repeat.