I would get a better idea of the types of foam that are moldable and fit into your characteristic expectations. There are soft urethanes and latex compounds that may work well, if not at least wear better than foams.
Here is a site that has some very decent mold making videos and liquid compounds to make the mold direct from your prototype. With a little molding ingenuity, you can mold this part directly on to your apparatus as the core. Which looks to be prosthetic?
That is an interesting project.
Doing a search for foam* + molding, I found this.