I was wondering if I could make something like the modified plastic castings in the previous post, but use polymer clay. I made a similar mold (only taller) using cardboard and cut pieces from the sides, so I could cut the clay after filling the mold.
Then I covered those openings with pieces of cardboard attached with adhesive tape. This helps in filling the mold.
When the mold was full, I removed those extra pieces.
Then I cut along the edges of the openings. That wasn't easy as the clay was a bit too soft even after a few hours in a fridge. Soft clay is easier when filling the mold, but makes cutting more difficult. The surfaces required some work after cutting to make them even.
Here are the soles after baking and removing the molds. I'm not quite happy with the shape, so I think these will have to be cut and sanded quite a bit. Still, I've got two soles of similar shape and size to work with, which is more than I could do without using molds.